Add OTLP metrics

[carloshernandez2]

Add OpenTelemetry Metrics Support

Summary

Extends the existing otlp exporter to support OpenTelemetry metrics in addition to logs. This enables efficient aggregation, querying, and analysis of REPL usage patterns.

Motivation

While the existing otlp exporter sends data as OpenTelemetry logs, metrics are better suited for:

• Aggregation and querying: Counters with labels enable efficient filtering and grouping
• Performance: Metrics are optimized for time-series data and queries
• Analysis: Session duration, operation frequency, and usage patterns can be calculated using PromQL
• Dashboards: Native support for visualization in Grafana and similar tools

Implementation

Unified OTLP Exporter

The otlp exporter now supports both logs and metrics:

• Exports a single counter: metrepl.events.total
• Uses labels/attributes for dimensions:
  • :metric - Event type (e.g., :event/op-completed)
  • :operation - REPL operation name (e.g., "eval") - only present for operation events
  • :correlation-id - Session identifier for grouping related events
  • System attributes: :hostname, :os, etc.

Users control which signals are exported via standard OpenTelemetry configuration:

• "otel.logs.exporter" - logs export control
• "otel.metrics.exporter" - metrics export control

Changes

Backward Compatibility

• Existing :otlp config key preserved (no breaking changes)
• Default configuration exports logs only (existing behavior)
• All existing configurations continue to work unchanged
• Users opt-in to metrics by configuring "otel.metrics.exporter": "otlp"

Testing

To test the metrics exporter, create .metrepl.edn:

{:exporters {:otlp {:enabled? true
                    :config {"otel.logs.exporter" "none"
                             "otel.metrics.exporter" "otlp"
                             "otel.exporter.otlp.metrics.protocol" "http/protobuf"
                             "otel.exporter.otlp.metrics.endpoint" "https://your-endpoint/metrics"}}}}

Tested with real project REPL sessions. Both logs and metrics successfully exported and visible in Grafana.

[ericdallo]

Thank you for the help!
I'm not sure it makes sense to add another exporter just because of a specific existing one config, I think we should keep using otlp and user can override its config to :

:config {"otel.logs.exporter"  "none"
     "otel.metrics.exporter" "otlp" 

WDYT?

[carloshernandez2]

I believe it makes sense. I initially thought it could be a bit awkward given the config structure (enabled flag and stuff) and that it might introduce coupling. But thinking it over I agree with you, the OTLP lib config structure looks flexible enough.

[ericdallo]

Looks good!