OTLP registry emitter

[jerbly]

Adds a new command: registry emit to send real OTLP messages to a receiver. This is may be useful in testing scenarios. Defaults to "http://localhost:4317" if the OpenTelemetry standard OTEL_EXPORTER_OTLP_ENDPOINT env var is unset.

With this command:

weaver registry emit

A single trace with all defined spans in the current Semantic Conventions are sent to the receiver:

Spans:

• Emit all defined spans for the registry in a single trace.
• Use standard OpenTelemetry libraries for real-world compatibility
• Write tests using opentelemetry-stdout to check output
• Generate examples from the resolved examples in the registry
• Use the correct span_kind
• Generate examples for template types (correctness issues prevent this)
• Adhere to conditionally_required clause (programmatically doing this, is not possible)

Metrics:

• Emit all defined metrics for the registry.