Trace SDK observability - span metrics

[pellared]

Common requirement as this is an experimental feature:

• To enable this feature the user has to set the OTEL_GO_X_SELF_OBSERVABILITY environment variable to the case-insensitive string value of true. If enabled, this instrumentation has to use the global MeterProvider.

Implement following self-observability metrics from https://github.com/open-telemetry/semantic-conventions/blob/v1.36.0/docs/otel/sdk-metrics.md for https://pkg.go.dev/go.opentelemetry.io/otel/sdk/trace:

• otel.sdk.span.live
• otel.sdk.span.started

Describe in the documentation the new feature in GoDoc. Here is an example of how metrics can be documented: https://github.com/open-telemetry/opentelemetry-go-contrib/blob/main/instrumentation/runtime/doc.go

Prototype for other SDK metrics for reference: #6393