otlpmetricgrpc: Keep metadata for gRPC in context

[mrasu]

When using otlpmetricgrpc, we can inject metadata dynamically with metadata.AppendToOutgoingContext. However when specifying headers with WithHeaders, it becomes impossible to add additional metadata.

For example, the code below sends additional metadata in the header

exp := otlpmetricgrpc.New(context.Background())

ctx := context.Background()
ctx = metadata.AppendToOutgoingContext(ctx, "dynamic-key", "value")
exp.Export(ctx, &metricdata.ResourceMetrics{}))

But when using WithHeader like below, no additional metadata will be sent

exp := otlpmetricgrpc.New(context.Background(), otlpmetricgrpc.WithHeaders("custom-key", "value"))

ctx := context.Background()
ctx = metadata.AppendToOutgoingContext(ctx, "dynamic-key", "value")
exp.Export(ctx, &metricdata.ResourceMetrics{}))

To eliminate this inconsistency, keep the metadata in the context and send them in the header.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.6%. Comparing base (bd88af9) to head (354ae01).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #5892     +/-   ##
=======================================
- Coverage   84.6%   84.6%   -0.1%     
=======================================
  Files        272     272             
  Lines      22861   22864      +3     
=======================================
+ Hits       19344   19346      +2     
- Misses      3172    3173      +1     
  Partials     345     345             

see 2 files with indirect coverage changes

[dmathieu]

We should do the same for the trace and log signals (in other PRs).