Review polish: fix stale exemplar context, dedupe rationales, close test gaps

- The exemplars docs described "the pinned opentelemetry-exporter-prometheus"
  and "Datasette's own Prometheus exporter" - context from demo/plugin work
  that is no longer part of this stack. Reworded to stand alone.
- Saturate a num_sql_threads=1 pool and assert the queue-depth gauge reads
  above zero - the headline alerting metric previously only had an absence
  test, and this also pins the private ThreadPoolExecutor._work_queue
  attribute it depends on.
- Pin error.type on the write path of db.client.operation.duration - the
  write wrappers time a different code path than the read one already tested.
- Isolate the non-threaded-mode gauge test from other live instances instead
  of comparing global observation counts, which a GC pass could shift.
- Halve the metrics banner, point conftest's meter note at it, compact the
  interrupted-counter call-site comment to a registry pointer, note why
  instrument and registry descriptions are separate strings, and stop
  calling the metric dimension a "later phase" now that metrics shipped.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012U7coQfVu8nK2R4q2mCULA

datasette/database.py

@@ -938,13 +938,9 @@ class Database:
                     if not timeout_expected:
                         span.set_status(Status(StatusCode.ERROR, str(e)))
                         span.record_exception(e)
-                        # A counter rather than only a span, because this is the
-                        # one thing an operator wants a rate and an alert on, and
-                        # spans under a 1% sampler cannot provide either. An
-                        # expected timeout - a caller that opted into a shorter
-                        # budget, like facet suggestion - is not counted, for the
-                        # same reason it is not a span error: it fires routinely
-                        # by design and would drown the signal this exists for.
+                        # Expected timeouts (a caller that opted into a shorter
+                        # budget, like facet suggestion) are not counted - see
+                        # the M_QUERIES_INTERRUPTED registry entry for why.
                         record_query_interrupted(self.name)
                     raise
                 except Exception as e: