enhancement: decouple livestore from metrics-generator by register a new metrics service

[javiermolinar]

What this PR does:
Currently the LiveStore registers the MetricsGenerator service for serving metrics queries. We need to decouple it first so we can have two different services. One for the pushing spans via grpc to the metrics generator and another one for serving queries.

The Metrics proto service was not used anywhere so we are reusing it.

From:

service MetricsGenerator {
  rpc PushSpans(PushSpansRequest) returns (PushResponse) {}
  rpc GetMetrics(SpanMetricsRequest) returns (SpanMetricsResponse) {}
  rpc QueryRange(QueryRangeRequest) returns (QueryRangeResponse) {}
}

To

service MetricsGenerator {
  rpc PushSpans(PushSpansRequest) returns (PushResponse) {}
}

service Metrics {
  rpc QueryRange(QueryRangeRequest) returns (QueryRangeResponse) {}
}

This will be done in a two phase deployment:

1. Register both services in the LiveStore.
2. Do a complete rollout
3. Swap the old service in the Queriers by the new one

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]

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve Metrics.SpanMetricsSummary RPC compatibility

Removing SpanMetricsSummary from the Metrics service is a wire/API breaking change for any existing gRPC clients generated from the previous tempo.proto: calls to /tempopb.Metrics/SpanMetricsSummary will fail with UNIMPLEMENTED after upgrading the server, even though the commit’s functional goal (adding a separate metrics service registration for live-store query-range) does not require dropping this RPC. Keeping the RPC and returning an explicit unimplemented response where needed would avoid breaking mixed-version or external clients.

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[javiermolinar]

tempopb.Metrics wasn’t registered anywhere before, so /tempopb.Metrics/SpanMetricsSummary was already effectively unavailable.