Add config endpoint

[MichelHollands]

This PR adds a /config endpoint which works the same as in Cortex. See the documentation for an explanation of the mode parameter.

Here's an example running it against Loki running in a Docker container built by the Makefile:

cortex % curl "http://localhost:3100/config?mode=diff"
compactor:
  shared_store: filesystem
  working_directory: /loki/boltdb-shipper-compactor
ingester:
  chunk_idle_period: 1h0m0s
  chunk_retain_period: 30s
  chunk_target_size: 1048576
  lifecycler:
    address: 127.0.0.1
    final_sleep: 0s
    ring:
      kvstore:
        store: inmemory
      replication_factor: 1
  query_store_max_look_back_period: 1h21m0s
limits_config:
  reject_old_samples: true
  reject_old_samples_max_age: 168h0m0s
ruler:
  alertmanager_url: http://localhost:9093
  enable_api: true
  ring:
    kvstore:
      store: inmemory
  rule_path: /loki/rules-temp
  storage:
    local:
      directory: /loki/rules
    type: local
schema_config:
  configs:
  - chunks:
      period: 0s
      prefix: ""
      tags: {}
    from: "2020-10-24"
    index:
      period: 1d
      prefix: index_
      tags: {}
    object_store: filesystem
    row_shards: 16
    schema: v11
    store: boltdb-shipper
server:
  http_listen_port: 3100
storage_config:
  boltdb_shipper:
    active_index_directory: /loki/boltdb-shipper-active
    cache_location: /loki/boltdb-shipper-cache
    shared_store: filesystem
  filesystem:
    directory: /loki/chunks
cortex %

Checklist

• Documentation added
• Tests updated

[codecov-io]

Codecov Report

Merging #3228 (929f909) into master (0208071) will increase coverage by 0.01%.
The diff coverage is 57.69%.

@@            Coverage Diff             @@
##           master    #3228      +/-   ##
==========================================
+ Coverage   63.20%   63.22%   +0.01%     
==========================================
  Files         197      198       +1     
  Lines       16685    16763      +78     
==========================================
+ Hits        10546    10598      +52     
- Misses       5184     5197      +13     
- Partials      955      968      +13     

Impacted Files	Coverage Δ
pkg/loki/loki.go	0.00% <0.00%> (ø)
pkg/loki/config_handler.go	61.64% <61.64%> (ø)
pkg/querier/queryrange/limits.go	91.66% <0.00%> (-4.17%)	⬇️
pkg/logql/evaluator.go	89.87% <0.00%> (-0.36%)	⬇️
pkg/promtail/positions/positions.go	58.51% <0.00%> (+11.70%)	⬆️

[cyriltovena]

LGTM Nice one !