chore(otel): add support for http/protobuf OTLP metrics export

[milas]

Motivation

Add support for http/protobuf in addition to gRPC for OpenTelemetry OTLP metrics export.

This is the default OpenTelemetry SDKs use nowadays, and there is a defined OTEL_..._PROTOCOL env var pattern as well. (gRPC is preserved as a default in Argo Workflows Controller for backwards compatibility.)

Modifications

During telemetry initialization, look for the OTEL_EXPORTER_OTLP_METRICS_PROTOCOL or OTEL_EXPORTER_OTLP_PROTOCOL environment variables.

If unset or explicitly set to grpc, use gRPC exporter.
If has http/ prefix (e.g. http/protobuf or http/json), use HTTP exporter.

NOTE: This is technically different than the "modern" OTel SDK default, which prefers http/protobuf, but that'd be a breaking change from the Argo Workflows perspective, so HTTP can be opted-in to by setting the variable.

Verification

Local regression tests + familiarity with OpenTelemetry SDK behaviors.

Documentation

Added note about gRPC being default to docs/metrics.md + explanation for how to use HTTP.

[Joibel]

Thanks for doing this. I was aware it needed doing but hadn't gotten round to it.

[Copilot]

Pull Request Overview

This PR adds support for HTTP/protobuf OTLP metrics export in addition to the existing gRPC support. The implementation checks environment variables (OTEL_EXPORTER_OTLP_METRICS_PROTOCOL or OTEL_EXPORTER_OTLP_PROTOCOL) to determine which protocol to use, defaulting to gRPC for backwards compatibility.

Key Changes:

• Added protocol detection logic to support both gRPC and HTTP exporters based on environment variables
• Added the otlpmetrichttp dependency to enable HTTP-based OTLP export
• Updated documentation to explain the default gRPC behavior and how to opt into HTTP

Reviewed Changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 1 comment.

File	Description
util/telemetry/metrics.go	Added protocol selection logic to choose between gRPC and HTTP OTLP exporters based on environment variables
go.mod	Added dependency for go.opentelemetry.io/otel/exporters/otlp/otlpmetric/otlpmetrichttp v1.36.0
go.sum	Added checksums for the new HTTP exporter dependency
docs/metrics.md	Documented that gRPC is the default and explained how to switch to HTTP using environment variables

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[Copilot]

"GRPC" should be styled as "gRPC" (lowercase g, uppercase RPC) to match the standard capitalization used throughout the codebase and industry standard.

Suggested change

	By default, GRPC is used; to switch to HTTP, set either the `OTEL_EXPORTER_OTLP_PROTOCOL` or `OTEL_EXPORTER_OTLP_METRICS_PROTOCOL` environment variable to `http/protobuf`.
	By default, gRPC is used; to switch to HTTP, set either the `OTEL_EXPORTER_OTLP_PROTOCOL` or `OTEL_EXPORTER_OTLP_METRICS_PROTOCOL` environment variable to `http/protobuf`.

[argo-cd-cherry-pick-bot]

❌ Cherry-pick failed for 3.7. Please check the workflow logs for details.

[argo-cd-cherry-pick-bot]

❌ Cherry-pick failed for 3.6. Please check the workflow logs for details.