otlp: add suport for lazy deserializing otel protobuf messages

[ldufr]

This enable support for lazy deserializing of OTLP protobuf messages. By default nothing is actually changed, the added Unmarshal on a message that wasn't lazy deserializing simply return what was deserialized and can't fail.

Importantly, this PR changes the OpenTelemetry pdata package to a fork https://github.com/grafana/opentelemetry-collector, but there is an open pull request to upstream the changes to pdata, see open-telemetry/opentelemetry-collector#14371

Related: grafana/mimir#13962

Does this PR introduce a user-facing change?

NONE

---

Note

Introduces lazy deserialization for OTLP metrics and adopts a forked pdata to enable it.

• Replaces go.opentelemetry.io/collector/pdata with github.com/grafana/opentelemetry-collector/pdata via go.mod replace; updates go.sum accordingly
• In metrics_to_prw.go, reads metrics with metricSlice.Unmarshal(k, &metric) instead of At(k), adding error handling while keeping downstream processing unchanged

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[jesusvazquez]

LGTM

[aknuds1]

Looks good for what it is, but I don't think we want to merge this into mimir-prometheus yet, right. I think we for now want to test your Mimir branch instead?

[ldufr]

@aknuds1 Just to be clear, I did run the code in a dev for a week. Should we re-run a new test before merging that and going through the normal deployment process?