## Problem
The page service logic asserts that a tracing span is present with
tenant/timeline/shard IDs. An initial gRPC page service implementation
thus requires a tracing span.
Touches https://github.com/neondatabase/neon/issues/11728.
## Summary of changes
Adds an `ObservabilityLayer` middleware that generates a tracing span
and decorates it with IDs from the gRPC metadata.
This is a minimal implementation to address the tracing span assertion.
It will be extended with additional observability in later PRs.
8d7ed2a4ee