Update OtlpGrpcClientOptions to populate options for gRPC client sharing

[ML-dev-crypto]

Fixes #4239

OtlpGrpcClientOptions previously only had a = default constructor,
leaving every option at null/zero regardless of environment variables
or otel-spec defaults. This made it impossible to build a standalone
OtlpGrpcClient with spec-compliant defaults for sharing across the
trace/metric/log exporters, as described in the issue.

Changes:

• Added signal-independent (no TRACES_/METRICS_/LOGS_ prefix) env-var
  accessors to otlp_environment.h, e.g. GetOtlpDefaultGrpcClientEndpoint().
  These are inline, defined in the header rather than
  otlp_environment.cc, so that otlp_grpc_client (a lean transport-only
  target) doesn't need to depend on otlp_recordable and its
  sdk/logs, sdk/metrics, sdk/trace dependencies just to read an env var.
• OtlpGrpcClientOptions now has a real default constructor (in a new
  otlp_grpc_client_options.cc) that populates from those accessors,
  and a void* constructor that skips populating, for parity with the
  existing per-signal options classes.
• OtlpGrpcExporterOptions / OtlpGrpcMetricExporterOptions /
  OtlpGrpcLogRecordExporterOptions each gained a constructor taking a
  const OtlpGrpcClientOptions&, copying the client-fixed fields
  (endpoint, SSL/mTLS fields, credentials, user_agent, channel_arguments,
  max_threads, compression, retry policy) and overriding only the fields
  that may still differ per signal (timeout, metadata,
  max_concurrent_requests) — per the table in the issue.
• Build files updated for the new source file and one new Bazel dep
  (//sdk/src/common:env_variables); no new library targets, no change
  to the existing dependency graph shape.
• New unit test otlp_grpc_client_options_test.cc covering: default
  env-var population, the void* no-defaults path (including the
  derived classes' void* constructors), fixed-field copying from a
  shared client, and signal-specific env-var precedence over the
  shared client's generic value.

Tested: compiled and unit-tested with both GCC/libstdc++ and
MSVC/STL; clang-format applied.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: ML-dev-crypto / name: Ansh Rai (4998b72)

[codecov]

Codecov Report

❌ Patch coverage is 97.26027% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.15%. Comparing base (4e20f69) to head (4998b72).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...de/opentelemetry/exporters/otlp/otlp_environment.h	95.88%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4249      +/-   ##
==========================================
+ Coverage   77.92%   78.15%   +0.24%     
==========================================
  Files         439      440       +1     
  Lines       18612    18754     +142     
==========================================
+ Hits        14502    14656     +154     
+ Misses       4110     4098      -12     

Files with missing lines	Coverage Δ
...elemetry/exporters/otlp/otlp_grpc_client_options.h	100.00% <ø> (ø)
...emetry/exporters/otlp/otlp_grpc_exporter_options.h	100.00% <ø> (ø)
...rters/otlp/otlp_grpc_log_record_exporter_options.h	100.00% <ø> (ø)
...exporters/otlp/otlp_grpc_metric_exporter_options.h	100.00% <ø> (ø)
exporters/otlp/src/otlp_grpc_client_options.cc	100.00% <100.00%> (ø)
exporters/otlp/src/otlp_grpc_exporter_options.cc	100.00% <100.00%> (+19.24%)	⬆️
.../otlp/src/otlp_grpc_log_record_exporter_options.cc	100.00% <100.00%> (+19.24%)	⬆️
...ters/otlp/src/otlp_grpc_metric_exporter_options.cc	100.00% <100.00%> (+21.43%)	⬆️
...de/opentelemetry/exporters/otlp/otlp_environment.h	96.30% <95.88%> (-3.70%)	⬇️

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[ML-dev-crypto]

Thanks for the thorough review, @owent! I'll address all of these comments in a follow-up commit, including the export annotation, constructor initialization cleanup, shared environment variable definitions, removal of the duplicated header parsing logic, moving the new environment helpers out of the header, reusing the base constructor for shared client initialization, and preserving the shared client's max_concurrent_requests value. I'll push everything together.

[owent]

Before C++17, a constexpr variable declared at namespace scope may have linkage symbol, so it emits an exported symbol. Please declare these as static constexpr char to give them internal linkage and avoid leaking symbols.

[owent]

Just a suggestion, use 'nostd::string_view' here can slightly improve performance.

[owent]

Just a few minor non-blocking issues left — otherwise LGTM. Could you please resolve the conflicts?

[ML-dev-crypto]

@owent Thanks! Addressed the remaining comments, rebased onto the latest main, resolved the merge conflict, and pushed the updates.

[ML-dev-crypto]

@owent
After rebasing, the CI is failing with linker errors because the generic GetOtlpDefaultGrpcClient*() helpers are currently defined in otlp_environment.cc, which isn't linked into the otlp_grpc_client target. I looked into a few possible approaches (moving the helpers, introducing a shared utility, etc.), but each would require changes to the target structure. Could you please take a look and let me know which direction you would recommend?