feat: Detect fields based on per-tenant configuration and put them into structured metadata at ingest time

[chaudum]

What this PR does / why we need it:

TBD

Example configuration:

limits_config:
  discover_generic_fields:
    trace_id:
      - "trace_id"
      - "TRACE_ID"
      - "traceID"
      - "TraceID"
    org_id:
      - "org_id"
      - "tenant_id"
      - "user_id"

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR