Add /status/overrides/{tenant} endpoint

[kvrhdn]

What this PR does:

⚠️ Depends on #3180 to be merged first, I've included some of its changes in here

Adds a new endpoint /status/overrides/{tenant} to fetch the overrides for a tenant.

With user-configurable overrides enabled:

$ curl http://localhost:3200/status/overrides/single-tenant
user_configurable_limits:
  metrics_generator:
    disable_collection: true
    collection_interval:
      duration: 30s
runtime_overrides:
  ingestion:
    rate_strategy: local
    rate_limit_bytes: 15000000
    burst_size_bytes: 20000000
    max_traces_per_user: 10000
  read:
    max_bytes_per_tag_values_query: 5000000
  compaction:
    block_retention: 1d
  metrics_generator:
    processor:
      span_metrics:
        dimensions:
        - foo
    ingestion_time_range_slack: 0s
  global:
    max_bytes_per_trace: 5000000

With user-configurable overrides disabled:

$ curl http://localhost:3200/status/overrides/single-tenant
ingestion:
  rate_strategy: local
  rate_limit_bytes: 15000000
  burst_size_bytes: 20000000
  max_traces_per_user: 10000
read:
  max_bytes_per_tag_values_query: 5000000
compaction:
  block_retention: 1d
metrics_generator:
  processor:
    span_metrics:
      dimensions:
      - foo
  ingestion_time_range_slack: 0s
global:
  max_bytes_per_trace: 5000000

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[zalegrala]

This looks reasonable to me. Would you like to add a test?

[kvrhdn]

Recent changes:

• removed the TODO and added /status/overrides as an alias for /status/runtime_config, both will be valid
• added a test case for getRuntimeOverridesFor
  • this caused a large amount of changes because I needed to expose the prometheus.Registerer as parameter, the runtime config manager uses DefaultRegisterer which caused double registration panics