fix(tempo): enable local-blocks so traceql metrics queries work

[skyf0l]

What

Enables Tempo's metrics-generator with the local-blocks processor only, and
points its WAL and trace storage at the existing PVC.

Why

Grafana's Tempo explore page issues a TraceQL metrics query by default:

{nestedSetParent<0 && true && resource.service.name != nil} | rate() by(resource.service.name)

| rate() is computed by the metrics-generator, not by Prometheus. #242 left the
generator off, so Tempo logged "metrics-generator is not configured" at boot and
every such panel returned 500 with error finding generators: empty ring.

Only local-blocks is enabled — it answers TraceQL metrics from the generator's
own blocks. span-metrics and service-graphs stay off: they remote-write a
per-span-name and per-edge series set into Prometheus, and Thanos block size here
scales with distinct series, which already needed trimming once (#225).

filter_server_spans is set to false so rate() over an arbitrary TraceQL
selector matches what the trace view shows, rather than only server-kind spans.
Storage paths live under /var/tempo, which is the existing PVC, so generator
state survives a restart without a second volume.

Verification

• Ran grafana/tempo:2.10.8 against the rendered config in Docker: modules
  metrics-generator-ring and metrics-generator both start and the instance
  registers in the ring. That is the direct cause of the reported error.
• Note that -config.verify=true is NOT a meaningful gate on this version — it
  returned 0 for a deliberately invalid processor name and for an unknown config
  key, so the check above is a real run, not a lint.
• make template: "all charts rendered OK". make validate-schema: 519 valid, 0
  invalid. prettier clean.

Risk

The generator receives a copy of every span in-process, so Tempo's memory use
rises; the pod limit stays at 1Gi and is worth watching once real traces flow.
flush_to_storage is left at its default of false, so this adds no R2 objects —
TraceQL metrics are answered from local blocks only, which bounds how far back
those queries can reach.

Rollback is reverting the commit; the generator's on-disk state is scratch.