stackdriver: further improvements

[djc]

• Avoid spawning inside the library
• Use a concrete error type
• Generate protobuf bindings in a test (avoids build script + dependencies, makes generated code visible to the IDE)

[codecov]

Codecov Report

Merging #703 (c873824) into main (0d116a0) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #703      +/-   ##
==========================================
+ Coverage   71.39%   71.40%   +0.01%     
==========================================
  Files         101      101              
  Lines        8564     8564              
==========================================
+ Hits         6114     6115       +1     
+ Misses       2450     2449       -1     

Impacted Files	Coverage Δ
opentelemetry/src/sdk/metrics/controllers/push.rs	83.33% <0.00%> (+3.33%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0d116a0...c873824. Read the comment docs.