Document audit telemetry

Author: andreasohlund

docs/telemetry.md

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+# Telemetry
+
+Instances can configured to emit telemetry to aid when performance testing or troubleshooting performance related issues.
+
+## Error
+
+Setting `ServiceControl/PrintMetrics` to `true` will print metrics to the logs at `INFO` level.
+
+## Audit
+
+Set `ServiceControl.Audit/OtlpEndpointUrl` to a valid [OTLP endpoint url](https://opentelemetry.io/docs/specs/otel/protocol/exporter/#configuration-options). Only GRPC endpoints are supported at this stage.
+
+It's recommended to use a local [OTEL Collector](https://opentelemetry.io/docs/collector/) to collect, batch and export the metrics to the relevant observability backend being used.
+
+The following metrics are available:
+
+### Ingestion
+
+- `sc.audit.ingestion.count` - Successful ingested audit message count
+- `sc.audit.ingestion.retry` - Retried audit message count
+- `sc.audit.ingestion.failed` - Failed audit message count
+- `sc.audit.ingestion.duration` - Audit message processing duration (in milliseconds)
+- `sc.audit.ingestion.message_size` - Audit message body size (in kilobytes)
+- `sc.audit.ingestion.forwarded_count` - Forwarded audiit messages count
+
+### Batching
+
+- `sc.audit.ingestion.batch_duration` - Batch processing duration (in milliseconds)
+- `sc.audit.ingestion.batch_size` - Batch size (number of messages)
+- `sc.audit.ingestion.consecutive_batch_failures` - Consecutive batch failures
+
+### Storage
+
+- `sc.audit.ingestion.audits_count` - Stored audit message count
+- `sc.audit.ingestion.sagas_count` - Stored sagas message count
+- `sc.audit.ingestion.commit_duration` - Storage unit of work commit duration (in milliseconds)
+
+## Monitoring
+
+No telemetry is currently available.