Schema URL inconsistently exported

[samin36]

Describe your environment
Opentelemetry-cpp v1.12.0 on Ubuntu 18

Steps to reproduce
Specify schema_url to the GetMeter, GetTracer, and Resource::Create functions and set up Tracer/Meter providers to export using the OTLP HTTP exporters.

What is the expected behavior?
The exported metrics and traces should contain the schema_url.

What is the actual behavior?
Only the exported traces contain the schema_url. The exported metric data does not contain the schema_url.

Additional context
I verified this by using the logging exporter in the Opentelemetry-Collector, which prints the metric/tracing data. I also set up a dummy HTTP server to log all incoming data at /v1/metrics and /v1/traces endpoint and only saw the schema_url for the tracing data.

[lalitb]

Thanks for the issue. The schema_url is not getting populated for metrics here: OtlpMetricUtils::PopulateResourceMetrics().

Need to add

resource_metrics->set_schema_url(scope_metrics.scope_->GetSchemaUrl());

in above-mentioned method.

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