fix(bigtable): respect GOOGLE_CLOUD_CPP_OPENTELEMETRY_TRACING

[dbolduc]

Bigtable data plane does not use PopulateCommonOptions(...) so we copy the handling of GOOGLE_CLOUD_CPP_OPENTELEMETRY_TRACING:

google-cloud-cpp/google/cloud/internal/populate_common_options.cc

Lines 67 to 70 in c7c14df

	e = GetEnv("GOOGLE_CLOUD_CPP_OPENTELEMETRY_TRACING");
	if (e && !e->empty()) {
	opts.set<OpenTelemetryTracingOption>(true);
	}

---

This change is 

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.26%. Comparing base (95fce04) to head (84d028c).
Report is 17 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13748      +/-   ##
==========================================
- Coverage   93.26%   93.26%   -0.01%     
==========================================
  Files        2229     2229              
  Lines      193104   193114      +10     
==========================================
+ Hits       180105   180112       +7     
- Misses      12999    13002       +3     

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