Add a plugin telemetry kit: public registry API, linked_root_span_kwargs, test helpers, docs

A survey of five plugin OTel plans (datasette-paper, -agent, -litestream,
-accounts, -cron) found every one hand-copying the same core machinery:
the registry classes, the conformance-test harness, the pytest fixtures,
the bucket boundaries and the detached-root-with-Link recipe. This makes
that machinery importable instead:

- The registry classes are documented public API. Attribute gains
  values= (a closed enum the conformance helpers enforce - what makes an
  attribute safe as a metric dimension); SpanName gains prefix=True for
  span families like "chat {model}" whose names share a fixed prefix,
  matched by span_for() after exact names. span_for()/attribute helpers
  accept a spans= tuple so plugin registries can use them.
- datasette.telemetry.linked_root_span_kwargs(): the root-span-with-Link
  shape for work a request caused without containing - background jobs,
  scheduled ticks, block=False writes. Core's own write thread now uses
  it instead of building the kwargs inline.
- datasette.telemetry_testing: the session provider fixtures, otel_spans
  / otel_metrics, a two-way registry conformance checker (including enum
  and prefix handling, filtered by instrumentation scope) and an
  assert_package_never_imports_sdk() guard. Core's conftest now imports
  these instead of defining them, so the suite consumes the kit exactly
  as a plugin's would.
- New "Telemetry for plugin authors" docs page: scope discipline,
  registry usage, privacy/cardinality rules, named-callable guidance,
  request_span(), the background root-with-link convention (one root per
  tick, always emitted), provider-ordering facts and known caveats.
  request_span() is now documented public API.

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

docs/telemetry_doc.py

@@ -15,7 +15,11 @@ def _attribute_lines(cog, attributes):
     cog.out("    Attributes:\n\n")
     for attribute in attributes:
         suffix = " *(optional)*" if attribute.optional else ""
-        cog.out(f"    - ``{attribute}``{suffix} - {attribute.description}\n")
+        line = f"    - ``{attribute}``{suffix} - {attribute.description}"
+        if attribute.values is not None:
+            rendered = ", ".join(f"``{value}``" for value in sorted(attribute.values))
+            line += f" One of: {rendered}."
+        cog.out(line + "\n")
     cog.out("\n")