Support for gRPC client spans

[grcevski]

This PR adds support for gRPC client spans, much like we support HTTP client spans. We use the same logic to nest the gRPC spans within server gRPC spans or server HTTP spans.

Outstanding work:

• More unit tests
• Integration tests for both HTTP and gRPC client

Closes #9

[mariomac]

Amazing!

[codecov-commenter]

Codecov Report

Merging #104 (5a0d32d) into main (8d3e69b) will increase coverage by 0.29%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##             main     #104      +/-   ##
==========================================
+ Coverage   73.70%   73.99%   +0.29%     
==========================================
  Files          20       20              
  Lines        1464     1496      +32     
==========================================
+ Hits         1079     1107      +28     
- Misses        292      293       +1     
- Partials       93       96       +3     

Flag	Coverage Δ
integration-test	59.85% <31.81%> (+0.88%)	⬆️
unittests	47.37% <52.27%> (-0.52%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pkg/goexec/structmembers.go	75.45% <ø> (ø)
pkg/export/otel/metrics.go	76.66% <62.50%> (+8.13%)	⬆️
pkg/export/otel/traces.go	76.92% <68.00%> (-3.66%)	⬇️
pkg/ebpf/grpc/grpc.go	100.00% <100.00%> (ø)
pkg/transform/spanner.go	83.33% <100.00%> (+0.83%)	⬆️

... and 1 file with indirect coverage changes

[grcevski]

Thanks Mario!