Race condition in set_tracer_provider()

[aabmass]

An unlikely race condition exists where multiple threads can set the global tracer provider at once, bypassing the check that the global tracer provider is only set once (code). This code needs a lock to prevent this condition:

opentelemetry-python/opentelemetry-api/src/opentelemetry/trace/__init__.py

Lines 487 to 491 in c9b18c6

	if _TRACER_PROVIDER is not None:
	logger.warning("Overriding of current TracerProvider is not allowed")
	return
	_TRACER_PROVIDER = tracer_provider

Describe your environment N/A

Steps to reproduce
Difficult to reproduce (race condition)

What is the expected behavior?
You can only configure the global tracer provider exactly once. All existing ProxyTracer instances should now proxy through this single global.

What is the actual behavior?
Global tracer provider could be set more than once if multiple threads attempt to do so. That could lead some ProxyTracer instances to cache a _real_tracer from a stale tracer provider and that Telemetry is lost or misconfigured.

Additional context
Ran into this while working on proxying for metrics.