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

tests/conftest.py

@@ -123,12 +123,10 @@ def _otel_meter_provider():
     Install a real OTel SDK MeterProvider + InMemoryMetricReader once per
     process.
 
-    Unlike the tracer, ordering is not load-bearing here: `_ProxyMeter` and
-    the `_ProxyInstrument`s it hands out forward to a provider installed
-    *after* they were created, whereas `ProxyTracer` permanently caches the
-    first concrete tracer it resolves. This fixture is still session-scoped
-    and autouse for symmetry, and so that a single reader collects for the
-    whole run.
+    Unlike the tracer, ordering is not load-bearing here - see the metrics
+    banner in `datasette/telemetry.py` for the `_ProxyMeter`-vs-`ProxyTracer`
+    difference. This fixture is still session-scoped and autouse for
+    symmetry, and so that a single reader collects for the whole run.
 
     DELTA temporality is chosen for counters and histograms so that each
     collection reports only what happened since the previous one. With the