fix: add mutex to internal trace otel test variable

[harshachinta]

Fixes #9388

The variable OpenTelemetryTracingEnabled is set by client library for testing purposes. However, there is a datarace condition that occurs when this variable is getting read in trace package and set in client library.
Sigma link
Interestingly this is not reproducible on local with -race argument and not in presubmits too. But it failed in continuous build.

Stack trace

=== RUN   TestSpannerTracesWithOpenTelemetry
==================
WARNING: DATA RACE
Write at 0x0000026759b0 by goroutine 536:
  cloud.google.com/go/spanner/test.TestSpannerTracesWithOpenTelemetry()
      /tmpfs/src/google-cloud-go/spanner/test/opentelemetry/test/ot_traces_test.go:37 +0x6b
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:1595 +0x238
  testing.(*T).Run.func1()
      /usr/local/go/src/testing/testing.go:1648 +0x44

Previous read at 0x0000026759b0 by goroutine 500:
  cloud.google.com/go/internal/trace.TracePrintf()
      /tmpfs/src/google-cloud-go/internal/trace/trace.go:190 +0x72
  cloud.google.com/go/spanner.(*sessionClient).executeBatchCreateSessions()
      /tmpfs/src/google-cloud-go/spanner/sessionclient.go:310 +0x1c5a
  cloud.google.com/go/spanner.(*sessionClient).batchCreateSessions.func2()
      /tmpfs/src/google-cloud-go/spanner/sessionclient.go:234 +0x8c

Goroutine 536 (running) created at:
  testing.(*T).Run()
      /usr/local/go/src/testing/testing.go:1648 +0x82a
  testing.runTests.func1()
      /usr/local/go/src/testing/testing.go:2054 +0x84
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:1595 +0x238
  testing.runTests()
      /usr/local/go/src/testing/testing.go:2052 +0x896
  testing.(*M).Run()
      /usr/local/go/src/testing/testing.go:1925 +0xb57
  main.main()
      _testmain.go:57 +0x2bd

Goroutine 500 (finished) created at:
  cloud.google.com/go/spanner.(*sessionClient).batchCreateSessions()
      /tmpfs/src/google-cloud-go/spanner/sessionclient.go:234 +0x4ec
  cloud.google.com/go/spanner.(*sessionPool).growPoolLocked()
      /tmpfs/src/google-cloud-go/spanner/session.go:812 +0x187
  cloud.google.com/go/spanner.(*sessionPool).take()
      /tmpfs/src/google-cloud-go/spanner/session.go:1074 +0x788
  cloud.google.com/go/spanner.(*ReadOnlyTransaction).acquireSingleUse()
      /tmpfs/src/google-cloud-go/spanner/transaction.go:803 +0x553
  cloud.google.com/go/spanner.(*ReadOnlyTransaction).acquire()
      /tmpfs/src/google-cloud-go/spanner/transaction.go:780 +0x7b
  cloud.google.com/go/spanner.(*txReadOnly).ReadWithOptions()
      /tmpfs/src/google-cloud-go/spanner/transaction.go:228 +0x276
  cloud.google.com/go/spanner.(*txReadOnly).Read()
      /tmpfs/src/google-cloud-go/spanner/transaction.go:148 +0xb04
  cloud.google.com/go/spanner/test.TestOTMetrics_GFELatency()
      /tmpfs/src/google-cloud-go/spanner/test/opentelemetry/test/ot_metrics_test.go:288 +0xa4f
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:1595 +0x238
  testing.(*T).Run.func1()
      /usr/local/go/src/testing/testing.go:1648 +0x44
==================
    testing.go:1465: race detected during execution of test
--- FAIL: TestSpannerTracesWithOpenTelemetry (0.02s)
=== NAME  
    testing.go:1465: race detected during execution of test