refactor(resource-detector-gcp): migrate away from getEnv()

[pichlermarc]

Which problem is this PR solving?

We're planning to remove getEnv() as it does not scale well when everyone has to update @opentelemetry/core to get their env vars. For OTEL_* there will be a replacement utility function. For all others, when the package does not need to be browser-compatible (which seems to be the case here) using process.env directly should be fine.

Refs open-telemetry/opentelemetry-js#5443

Short description of the changes

This PR removes getEnv() with process.env and inlines the defaults from @opentelemetry/core's DEFAULT_ENVIRONMENT

[pichlermarc]

Note for reviewers: this previously was an empty string when it was unset - now it's undefined. The semantics should be the same other than that.

[pichlermarc]

Note for reviewers:

See here for previous defaults:

• NAMESPACE ->
  https://github.com/open-telemetry/opentelemetry-js/blob/15ba2d7da3bd6663ee6008e189f004bab93ff6fe/packages/opentelemetry-core/src/utils/environment.ts#L164
• HOSTNAME -> https://github.com/open-telemetry/opentelemetry-js/blob/15ba2d7da3bd6663ee6008e189f004bab93ff6fe/packages/opentelemetry-core/src/utils/environment.ts#L162
• CONTAINER_NAME -> https://github.com/open-telemetry/opentelemetry-js/blob/15ba2d7da3bd6663ee6008e189f004bab93ff6fe/packages/opentelemetry-core/src/utils/environment.ts#L88

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.40%. Comparing base (db8245a) to head (be6f4e7).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2705      +/-   ##
==========================================
- Coverage   92.40%   92.40%   -0.01%     
==========================================
  Files         171      171              
  Lines        8142     8140       -2     
  Branches     1649     1652       +3     
==========================================
- Hits         7524     7522       -2     
  Misses        618      618              

Files with missing lines	Coverage Δ
...resource-detector-gcp/src/detectors/GcpDetector.ts	96.22% <100.00%> (-0.14%)	⬇️