pageserver: fix duplicate shard_id in span (#6981)

## Problem

shard_id in span is repeated:
- https://github.com/neondatabase/neon/issues/6723

Closes: #6723

## Summary of changes

- Only add shard_id to the span when fetching a cached timeline, as it
is already added when loading an uncached timeline.

pageserver/src/page_service.rs

@@ -1115,7 +1115,10 @@ impl PageServerHandler {
         ctx: &RequestContext,
     ) -> Result<PagestreamBeMessage, PageStreamError> {
         let timeline = match self.get_cached_timeline_for_page(req) {
-            Ok(tl) => tl,
+            Ok(tl) => {
+                set_tracing_field_shard_id(tl);
+                tl
+            }
             Err(key) => {
                 match self
                     .load_timeline_for_page(tenant_id, timeline_id, key)
@@ -1140,9 +1143,6 @@ impl PageServerHandler {
             }
         };
 
-        // load_timeline_for_page sets shard_id, but get_cached_timeline_for_page doesn't
-        set_tracing_field_shard_id(timeline);
-
         let _timer = timeline
             .query_metrics
             .start_timer(metrics::SmgrQueryType::GetPageAtLsn);