Add toggle to inject the tenant ID to generated metrics

[yvrhdn]

What this PR does:

When a inject_tenant_id_as is set, the Tempo tenant ID is added to all metrics generated by the metrics-generator. E.g. if set to __tempo_tenant, metrics generated for tenant foo will all have the label __tempo_tenant="foo".

metrics_generator:
  registry:
    inject_tenant_id_as: "__tempo_tenant"

I made the config a string so users can pick a value that doesn't conflict with other labels.

This should make Tempo metrics-generator more useful with a single-tenant metrics backend.

Which issue(s) this PR fixes: Fixes https://github.com/grafana/tempo/issues/3630

Checklist

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