API v3alpha: ReceiverService

Parent: #5450

Summary

Define and implement receiver.v3alpha.ReceiverService, initially providing deterministic receiver discovery independently of notification observability.

The public proto should be reviewed first, followed by the handler implementation.

Proto PR

Define:

• Receiver identity and label messages using common.v3alpha where applicable.
• Receiver label filters.
• ListReceivers request and response messages.
• ListReceivers RPC.

Document ordering, filter semantics, missing configuration behavior, and whether pagination is necessary for the initial contract.

Implementation

• Return every configured receiver in deterministic configuration order, including canonical name and custom labels.
• Match the existing API meaning of configured receivers rather than returning only receivers with currently active notification pipelines.
• Support OR across receiver matcher sets and AND within each set.
• Support equality, inequality, regular-expression, negated regular-expression, and missing-label behavior through shared matcher semantics.
• Clone label maps before returning them.
• Return Unavailable when configuration has not loaded.
• Publish receiver/config metadata through the immutable reload snapshot.
• Register the service consistently with handler routing, health, reflection, and bounded instrumentation.
• Treat any tightening of receiver label-name validation as a separate configuration-parser change; the API must not silently reinterpret existing labels.

Acceptance criteria

• The receiver.v3alpha proto and generated code land before handler implementation.
• All configured receivers are returned in deterministic configuration order.
• Canonical names and custom labels are included without exposing mutable internal maps.
• Shared OR-of-AND matcher semantics are applied consistently.
• Not-ready configuration returns Unavailable.
• Configuration reload atomically updates receiver results.
• Health, reflection, and bounded instrumentation include ReceiverService.
• Ginkgo/Gomega coverage includes ordering, empty/not-ready config, reload, name/custom labels, missing labels, regex, negation, and invalid matchers over supported transports.

Dependencies and related work

• Shared contracts and immutable reload snapshots: #5479.
• Connect registration, request policy, and TLS coverage: #5478.
• Receiver labels were introduced by #4963.