Remove prometheus-metrics-shaded-protobuf dependency from the agent

[laurit]

@trask WDYT?

[trask]

I'm not sure we can do this, it looks like exposition format is controlled by the client scraper via the Accept http header:

https://github.com/prometheus/client_java/blob/fc583c2e5813d4ab510f3464f7bf5d2f9b4d3ce2/prometheus-metrics-exposition-formats/src/main/java/io/prometheus/metrics/expositionformats/ExpositionFormats.java#L34-L36

[laurit]

I'm not sure we can do this, it looks like exposition format is controlled by the client scraper via the Accept http header

Thanks. https://opentelemetry.io/docs/specs/otel/metrics/sdk_exporters/prometheus/ doesn't say that the protobuf format must be supported. As far as I can tell support for the protobuf format was added not too long ago with open-telemetry/opentelemetry-java#6015

[trask]

added not too long ago with open-telemetry/opentelemetry-java#6015

this part is interesting:

In Prometheus, exponential histograms require the Prometheus protobuf exposition format.

[laurit]

Prometheus seems to support OTLP https://prometheus.io/docs/guides/opentelemetry/

[trask]

@fstab any thoughts?

[fstab]

In Prometheus, exponential histograms require the Prometheus protobuf exposition format.

Yes, exponential histograms are only supported in Prometheus protobuf format, not in Prometheus text format. There's work underway to specify a text representation, but that's not implemented yet.

If a Prometheus scraper runs with exponential histogram support enabled, it will signal protobuf support in the HTTP Accept: header, and then the Prometheus exporter will expose protobuf format, including exponential histograms.

I don't think this should be removed, otherwise support for exponential histograms would be lost.

[fstab]

Also, as the protobuf dependency is shaded into the io.prometheus package there should not be a conflict with the protobuf version used by the OTel SDK itself.

[laurit]

I don't think this should be removed, otherwise support for exponential histograms would be lost.

I created this PR under the assumption that text format can represent everything. I'll close it as we don't wish to loose this functionality.

Also, as the protobuf dependency is shaded into the io.prometheus package there should not be a conflict with the protobuf version used by the OTel SDK itself.

Neither the otel sdk nor the agent depend on the protobuf lbrary. The size of protobuf library is bit less than 2mb which is roughly 9% of the agent size.