Register metrics and give histograms bucket boundaries suited to seconds

Both histograms declared unit="s" but inherited OpenTelemetry's default
boundaries, which are tuned for milliseconds - so every SQLite query
landed in the single (0, 5] second bucket and every quantile query
returned noise.

The boundaries are the semantic conventions' recommended set for
db.client.operation.duration plus 0.0001 and 0.0005 at the bottom, since
SQLite is in-process and many real queries take tens of microseconds.

(Adapted from 024f2029: that commit assumed the metrics were already in
telemetry_registry.py, which on this lineage held spans only - so this
commit also brings the MetricName registry machinery, the registry
entries for all eight phase-3 metrics, the cog-generated Metric
reference in internals.rst, and the datasette.operation attribute. The
template and facet histograms it also touched belong to phase 5 and are
not included.)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01F2h9ANGZ7paWSpqs5DUAcG

docs/telemetry_doc.py

@@ -35,3 +35,22 @@ def spans(cog):
         if span.kind != SpanKind.INTERNAL:
             cog.out(f"    Kind: ``{span.kind.name}``.\n\n")
         _attribute_lines(cog, span.attributes)
+
+
+def metrics(cog):
+    from datasette.telemetry_registry import METRICS
+
+    cog.out("\n")
+    for metric in METRICS:
+        cog.out(f"``{metric}``\n")
+        cog.out(f"    {metric.kind}, unit ``{metric.unit}``. {metric.description}\n\n")
+        if metric.buckets:
+            boundaries = ", ".join(f"``{boundary}``" for boundary in metric.buckets)
+            cog.out(f"    Bucket boundaries: {boundaries}.\n\n")
+        if metric.attributes:
+            cog.out("    Attributes:\n\n")
+            for attribute in metric.attributes:
+                cog.out(f"    - ``{attribute}`` - {attribute.description}\n")
+            cog.out("\n")
+        else:
+            cog.out("    No attributes.\n\n")