Consider exporting the tracer map in go.opentelemetry.io/contrib/internal/trace

[reggiemcdonald]

The current mock trace provider stores a map of the tracers created, but it isnt exported:

opentelemetry-go-contrib/internal/trace/mock_tracer.go

Lines 30 to 33 in 818b337

	type Provider struct {
	tracersLock sync.Mutex
	tracers map[string]*Tracer
	}

As we shift to using WithTraceProvider options (#213), it would be helpful to export the tracer map in the provider struct of the mocktrace package (go.opentelemetry.io/contrib/internal/trace).

As it stands, you cannot type cast the trace.Tracer returned from the mock provider to the concrete mocktrace.Tracer type.
Thus, we cannot access any of the convenience functions such as mocktrace.Tracer.EndedSpans()

[MrAlias]

Closing, stale.