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SCAL-308279 Add Grafana OTLP metrics sink (#126)

* SCAL-308279 Add Grafana OTLP metrics sink

Add the direct Grafana OTLP/HTTP sink for low-cardinality MCP service metrics.
This keeps PR 3 independent from the Analytics Engine sink so it can be
reviewed and merged in parallel.

- add a Grafana OTLP metrics sink that emits OTLP JSON over HTTP
- convert counters, gauges, and histograms into OTLP metric datapoints
- normalize OTLP endpoints to `/v1/metrics`
- support Grafana Cloud Basic auth and explicit OTLP auth headers
- wire request metrics to build the Grafana sink from env config
- test payload mapping, endpoint config, auth headers, and failure behavior

The sink sends delta sums and histograms because request-scoped observations
are emitted as per-flush deltas. Export failures are surfaced to the recorder,
where the existing metrics flush isolation logs and drops them without changing
business responses.

- `npm run lint`
- focused runtime metrics tests for Grafana sink and request metrics

* SCAL-308279 Aggregate OTLP metric datapoints

## Summary

Address review feedback for the Grafana OTLP payload shape and timestamp precision.
OTLP requires each metric to contain at most one datapoint for a given attribute set.

## What Changed

- aggregate same-metric and same-label observations before creating OTLP datapoints
- sum counters and histogram count, sum, and bucket counts per attribute set
- keep the latest gauge value per attribute set
- convert millisecond timestamps to nanoseconds with `BigInt` precision
- add regression coverage for duplicate attributes and fractional millisecond timestamps

## Validation

- `npm run lint`
- focused runtime metrics tests
- `npm test`

* SCAL-308279 Avoid counter bucket allocation

## Summary

Avoid allocating histogram bucket storage for non-histogram OTLP datapoints.

## Validation

- `npm run lint`
- `npm test -- --coverage.enabled=false test/metrics/runtime/grafana-otlp-sink.spec.ts`

* SCAL-308279 Hard... (continued)

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