feat(o11y): per-instance metrics via injectable MeterProvider, drop package globals (PIPE-1066)

[Dylan-M]

Proposed Change

Reworks the weaver-generated metrics: each generator and output now builds its own
instruments from an injected MeterProvider instead of package-global singletons. Every
generator and output constructor takes a TelemetrySettings and calls NewMetrics on
the bundle's provider. A nil provider falls back to the process global, preserving
standalone behavior.

Dropping the globals lets multiple blitz instances (embedded in one host) record to
separate providers without cross-talk.

How to validate

• Run a generator into an output:
  go run ./cmd/blitz --generator-type apache-common --generator-apache-common-rate 1s --output-type stdout --metrics-port 9100
• In another shell, confirm the generator and output series are present and climbing:
  curl -s localhost:9100/metrics | grep -E 'blitz_generator_|blitz_output_'.
• go test ./generator/... ./output/... covers per-instance metric construction and the
  failing-provider path.

Checklist

• Changes are tested
• CI has passed

[Dylan-M]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• fix(o11y): record output request + HEC ACK-poll latency in ms (PIPE-1404) #292
• feat(o11y): lifecycle startup-latency metrics via new runtime weaver registry (PIPE-1066) #291
• feat(o11y): bring stdout/nop outputs fully into the self-telemetry spine (PIPE-1066) #290
• feat(o11y): per-batch output send spans + HEC ACK-poll span (PIPE-1066) #289
• feat(o11y): wire fix generator into the self-telemetry spine (PIPE-1066) #288
• feat(o11y): self-logs via shared entry-point zap->OTel bridge + OTLP log export (PIPE-1066) #287
• feat(o11y): self-tracing via runtime spans, adapter emit spans, and OTLP trace export (PIPE-1066) #286
• feat(o11y): per-instance metrics via injectable MeterProvider, drop package globals (PIPE-1066) #285 👈 (View in Graphite)
• feat(embed): add TelemetrySettings provider bundle + NopTelemetry (PIPE-1066) #284
• main

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