feat(opentelemetry): add OpenTelemetry OTLP metrics exporter

[pakhomovld]

• Closes 💡 [REQUEST] - Add OpenTelemetry Exporter plugin #347

Summary

Add an OpenTelemetry metrics exporter plugin that sends SLO metrics via OTLP gRPC to any OpenTelemetry-compatible backend
(Grafana Mimir, Datadog, New Relic, etc.).

How it works

• Subclasses MetricsExporter (same pattern as Prometheus, Datadog exporters)
• Uses synchronous Gauge instruments (available since OTel SDK v1.22.0), which resolves the async gauge blocker mentioned
  in 💡 [REQUEST] - Add OpenTelemetry Exporter plugin #347
• Creates a per-call MeterProvider with OTLPMetricExporter for endpoint isolation
• SLO report labels map directly to OpenTelemetry metric attributes
• All metrics use the slo_ prefix (e.g., slo_error_budget_burn_rate)

Changes

• slo_generator/exporters/opentelemetry.py - New exporter
• setup.cfg - opentelemetry extras group
• Makefile - Added opentelemetry to install target
• docs/providers/opentelemetry.md-Provider documentation
• samples/config.yaml - Sample exporter config
• tests/unit/test_compute.py - Unit test with mocked OTLP exporter
• tests/unit/fixtures/exporters.yaml - Test fixture entry
• tests/unit/test_stubs.py - Test context variables

Test plan

• make unit - all tests pass including new test_export_opentelemetry
• pre-commit run --all-files - all hooks pass