impl(otel): copy service resource labels into metric labels

[IcySteam]

fixed #14823

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This change is 

[dbolduc]

I don't think the copying is necessary. Can we check for the known service labels and do the manipulations here:

google-cloud-cpp/google/cloud/opentelemetry/internal/time_series.cc

Lines 174 to 177 in 912533c

	for (auto& label : mr.labels) {
	(*resource.mutable_labels())[std::move(label.first)] =
	std::move(label.second);
	}

and add a TEST(ToMonitoredResource, AddsServiceLabels) { ... } in time_series_test.cc ?

[dbolduc]

Sorry, I am confused. Do we want the labels to end up getting added to the google.api.Metric https://github.com/googleapis/googleapis/blob/2d05911be5a468b556236bee537c91922f9c23a3/google/api/metric.proto#L286

... or to the google.api.MonitoredResource https://github.com/googleapis/googleapis/blob/2d05911be5a468b556236bee537c91922f9c23a3/google/api/monitored_resource.proto#L106

... or both?

unit tests would help clarify the intended behavior that we want.

[dbolduc]

/gcbrun

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.06%. Comparing base (aa062cf) to head (1775a3e).
Report is 27 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #14825   +/-   ##
=======================================
  Coverage   93.06%   93.06%           
=======================================
  Files        2319     2319           
  Lines      209023   209023           
=======================================
+ Hits       194529   194530    +1     
+ Misses      14494    14493    -1     

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