pageserver: add gRPC observability middleware (#12093)

## 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.

libs/utils/src/lib.rs

@@ -73,6 +73,7 @@ pub mod error;
 /// async timeout helper
 pub mod timeout;
 
+pub mod span;
 pub mod sync;
 
 pub mod failpoint_support;

libs/utils/src/span.rs

@@ -0,0 +1,19 @@
+//! Tracing span helpers.
+
+/// Records the given fields in the current span, as a single call. The fields must already have
+/// been declared for the span (typically with empty values).
+#[macro_export]
+macro_rules! span_record {
+    ($($tokens:tt)*) => {$crate::span_record_in!(::tracing::Span::current(), $($tokens)*)};
+}
+
+/// Records the given fields in the given span, as a single call. The fields must already have been
+/// declared for the span (typically with empty values).
+#[macro_export]
+macro_rules! span_record_in {
+    ($span:expr, $($tokens:tt)*) => {
+        if let Some(meta) = $span.metadata() {
+            $span.record_all(&tracing::valueset!(meta.fields(), $($tokens)*));
+        }
+    };
+}