sdk/trace: self-observability: batch span processor metrics (#6393)

Fixes https://github.com/open-telemetry/opentelemetry-go/issues/7005

Adds `otel.sdk.processor.span.queue.size`,
`otel.sdk.processor.span.queue.capacity`, and
`otel.sdk.processor.span.processed.count` metrics to the trace batch
span processor.

These are defined in
cb11bb9bac/docs/otel/sdk-metrics.md,
and are experimental. Because of this, metrics are behind the
OTEL_GO_X_SELF_OBSERVABILITY feature gate.

Given the feature is experimental, it always uses the global
meterprovider when enabled.

---------

Co-authored-by: Robert Pająk <pellared@hotmail.com>

sdk/trace/tracer.go

@@ -39,7 +39,7 @@ func (tr *tracer) initSelfObservability() {
 
 	tr.selfObservabilityEnabled = true
 	mp := otel.GetMeterProvider()
-	m := mp.Meter("go.opentelemetry.io/otel/sdk/trace",
+	m := mp.Meter(selfObsScopeName,
 		metric.WithInstrumentationVersion(sdk.Version()),
 		metric.WithSchemaURL(semconv.SchemaURL))