Prometheus exporter adds underscore suffix for metrics without unit

[dashpole]

Steps to reproduce

If a metric does not have a unit set, the prometheus exporter will add an extra _ suffix to the metric. I have only second-hand knowledge of the issue, and don't have reproduction steps.

What is the expected behavior?

If the unit is empty, the name should not have an underscore or unit suffix added.

What is the actual behavior?

The metric grpc.client.attempt.started without a unit turns into:

# TYPE grpc_client_attempt_started_ counter
grpc_client_attempt_started_{grpc_method="grpc.testing.TestService/UnaryCall",grpc_target="dns:///10.52.1.55:14285"} 10 1694819876572

I believe this is because of this line:

opentelemetry-cpp/exporters/prometheus/src/exporter_utils.cc

Line 45 in 0563a71

metric_family.name = sanitized + "_" + unit;

We append an underscore regardless of if the unit is empty or not.

[yashykt]

This was seen on gRPC C++'s OTel plugin, but the pattern of usage is nothing out of the ordinary from what I understand. Reference - https://github.com/grpc/grpc/blob/62521a889fe134859aed7bdf4ea0730534f15d09/src/cpp/ext/otel/otel_plugin.cc#L170

A Meter with the name "grpc" was created, and a counter instrument with the name "grpc.client.attempt.started" was created out of it.

The counter was incremented like so - https://github.com/grpc/grpc/blob/62521a889fe134859aed7bdf4ea0730534f15d09/src/cpp/ext/otel/otel_client_filter.cc#L122

[github-actions]

This issue was marked as stale due to lack of activity.