Add OpenTelemetry backend, exemplars, and integration tests

[gmbyapa]

Breaking Changes

• Counter.Count, Gauge.Count, Gauge.Set, and Observer.Observe now accept variadic ...RecordOption parameter. Existing callers are unaffected, but custom interface implementations must update their signatures.

What's Changed

• Add OTELReporter backend with support for counters, gauges, gauge functions, and histograms backed by OpenTelemetry SDK
• Add WithContext(ctx) RecordOption to propagate trace context as exemplars on metric recordings
• Add functional options pattern for OTELReporter (WithMeterProvider)
• Add exponential histogram support via AggregationBase2ExponentialHistogram
• Add integration tests with Testcontainers verifying three ingestion paths: OTLP direct push, OpenMetrics scrape, and PrometheusProto scrape
• Verify native histograms + exemplars work end-to-end through PrometheusProto scrape protocol
• Add doc comments to all interfaces, structs, and fields in reporter.go
• Add GitHub Actions CI workflow (build, vet, integration tests)
• Update README with usage docs, exemplar guide, and native histogram
  • exemplar setup instructions