doc: add mention of GOOGLE_CLOUD_CPP_TRACING_OPTIONS env var (googleapis/google-cloud-cpp-spanner#1448)
doc: add mention of GOOGLE_CLOUD_CPP_TRACING_OPTIONS env var (googleapis/google-cloud-cpp-spanner#1448)


googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp