otel remote_write fails when run with -race

[kirbyquerby]

When running the remote_write tests for the OpenTelemetry collector with the -race flag:

go test -race --tags=compliance -run "TestRemoteWrite/otel/.+" -v ./

The tests fail due to data races: https://gist.github.com/kirbyquerby/59e4e57d59ba131307fb3d9ac2a4e35d

This issue was found when adding these tests to the OpenTelemetry collector (open-telemetry/opentelemetry-collector-contrib/pull/5014), as the tests in the collector are run with -race.