Introduce overrides-exporter module to Loki

[JordanRushing]

What this PR does / why we need it:
This PR introduces a new module to Loki that will export tenant limits including overrides to further expand visibility into tenant configurations and behavior.

Examples runtime-config with overrides:

overrides:
  "user1":
    ingestion_burst_size_mb: 350000
    ingestion_rate_mb: 350000
    max_query_series: 1000

Running Loki with only the overrides-exporter target specified and a basic schema_config:

$ ./cmd/loki/loki -config.file=demo_schema_config.yaml -target=overrides-exporter -runtime-config.file=example_runtime.yaml -server.http-listen-port=8080

Exported metrics:

$ curl -sq localhost:8080/metrics | grep override                                                                                                                                                                                                                          
# HELP loki_overrides Resource limit overrides applied to tenants                                                                                                                                                                                                          
# TYPE loki_overrides gauge                                                                                                                                                                                                                                                
loki_overrides{limit_name="cardinality_limit",user="user1"} 100000                                                                                                                                                                                                         
loki_overrides{limit_name="creation_grace_period",user="user1"} 6e+11                                                                                                                                                                                                      
loki_overrides{limit_name="ingestion_burst_size_mb",user="user1"} 350000                                                                                                                                                                                                   
loki_overrides{limit_name="ingestion_rate_mb",user="user1"} 350000                                                                                                                                                                                                         
loki_overrides{limit_name="max_cache_freshness_per_query",user="user1"} 6e+10                                                                                                                                                                                              
loki_overrides{limit_name="max_chunks_per_query",user="user1"} 2e+06                                                                                                                                                                                                       
loki_overrides{limit_name="max_concurrent_tail_requests",user="user1"} 10                                                                                                                                                                                                  
loki_overrides{limit_name="max_entries_limit_per_query",user="user1"} 5000                                                                                                                                                                                                 
loki_overrides{limit_name="max_global_streams_per_user",user="user1"} 5000                                                                                                                                                                                                 
loki_overrides{limit_name="max_label_name_length",user="user1"} 1024                                                                                                                                                                                                       
loki_overrides{limit_name="max_label_names_per_series",user="user1"} 30                                                                                                                                                                                                    
loki_overrides{limit_name="max_label_value_length",user="user1"} 2048                                                                                                                                                                                                      
loki_overrides{limit_name="max_line_size",user="user1"} 0                                                                                                                                                                                                                  
loki_overrides{limit_name="max_queriers_per_tenant",user="user1"} 0                                                                                                                                                                                                        
loki_overrides{limit_name="max_query_length",user="user1"} 2.5956e+15                                                                                                                                                                                                      
loki_overrides{limit_name="max_query_lookback",user="user1"} 0                                                                                                                                                                                                             
loki_overrides{limit_name="max_query_parallelism",user="user1"} 32                                                                                                                                                                                                         
loki_overrides{limit_name="max_query_series",user="user1"} 1000                                                                                                                                                                                                            
loki_overrides{limit_name="max_streams_matchers_per_query",user="user1"} 1000                                                                                                                                                                                              
loki_overrides{limit_name="max_streams_per_user",user="user1"} 0                                                                                                                                                                                                           
loki_overrides{limit_name="min_sharding_lookback",user="user1"} 0                                                                                                                                                                                                          
loki_overrides{limit_name="per_stream_rate_limit",user="user1"} 3.145728e+06                                                                                                                                                                                               
loki_overrides{limit_name="per_stream_rate_limit_burst",user="user1"} 1.572864e+07                                                                                                                                                                                         
loki_overrides{limit_name="per_tenant_override_period",user="user1"} 1e+10                                                                                                                                                                                                 
loki_overrides{limit_name="reject_old_samples_max_age",user="user1"} 1.2096e+15                                                                                                                                                                                            
loki_overrides{limit_name="retention_period",user="user1"} 2.6784e+15                                                                                                                                                                                                      
loki_overrides{limit_name="ruler_evaluation_delay_duration",user="user1"} 0                                                                                                                                                                                                
loki_overrides{limit_name="ruler_max_rule_groups_per_tenant",user="user1"} 0                                                                                                                                                                                               
loki_overrides{limit_name="ruler_max_rules_per_rule_group",user="user1"} 0                                                                                                                                                                                                 
loki_overrides{limit_name="ruler_remote_write_queue_batch_send_deadline",user="user1"} 0                                                                                                                                                                                   
loki_overrides{limit_name="ruler_remote_write_queue_capacity",user="user1"} 0                                                                                                                                                                                              
loki_overrides{limit_name="ruler_remote_write_queue_max_backoff",user="user1"} 0                                                                                                                                                                                           
loki_overrides{limit_name="ruler_remote_write_queue_max_samples_per_send",user="user1"} 0                                                                                                                                                                                  
loki_overrides{limit_name="ruler_remote_write_queue_max_shards",user="user1"} 0                                                                                                                                                                                            
loki_overrides{limit_name="ruler_remote_write_queue_min_backoff",user="user1"} 0                                                                                                                                                                                           
loki_overrides{limit_name="ruler_remote_write_queue_min_shards",user="user1"} 0                                                                                                                                                                                            
loki_overrides{limit_name="ruler_remote_write_timeout",user="user1"} 0                                                                                                                                                                                                     
loki_overrides{limit_name="split_queries_by_interval",user="user1"} 0 

Which issue(s) this PR fixes:
Closes #4389

Special notes for your reviewer:
N/A

Checklist

• Documentation added
• Tests updated

[JordanRushing]

@KMiller-Grafana QQ: any opinions on where to place new documentation for the overrides-exporter target for Loki? For reference here is the comparable doc for Cortex.

[dannykopping]

Looks great! A couple small notes

[dannykopping]

I like the reflection approach, personally. If security or cardinality becomes a concern, we could always add a denylist or allowlist mechanism to reduce them. I think the benefit of avoiding drift outweighs the potential risk here.