Add optional OpenTelemetry integration via kaya-otel

Adds an OTEL_ENABLED opt-in that assembles kaya-otel's OTelMixin into
the app, exporting HTTP/WebSocket traces and request metrics to an
OTLP/HTTP collector. Configured through OTEL_SERVICE_NAME,
OTEL_EXPORTER_OTLP_ENDPOINT, OTEL_EXPORTER_OTLP_HEADERS and
OTEL_EXCLUDED_PATHS (defaults to the /api/health probe endpoint).

kaya-otel is an optional 'otel' extra imported lazily, so default
installs and the test suite do not need the OpenTelemetry packages.
The kaya dependencies are bumped to 0.0.4, which kaya-otel requires.

server/requirements.txt

@@ -50,7 +50,7 @@ idna==3.19
     #   httpx
 iso8601==2.1.0
     # via tortoise-orm
-kaya-core==0.0.3
+kaya-core==0.0.4
     # via
     #   kaya-cors
     #   kaya-oidc
@@ -58,20 +58,20 @@ kaya-core==0.0.3
     #   kaya-rsgi
     #   kaya-session
     #   tavolo (pyproject.toml)
-kaya-cors==0.0.3
+kaya-cors==0.0.4
     # via tavolo (pyproject.toml)
-kaya-oidc==0.0.3
+kaya-oidc==0.0.4
     # via tavolo (pyproject.toml)
-kaya-openapi==0.0.3
+kaya-openapi==0.0.4
     # via tavolo (pyproject.toml)
-kaya-rsgi==0.0.3
+kaya-rsgi==0.0.4
     # via tavolo (pyproject.toml)
-kaya-session==0.0.3
+kaya-session==0.0.4
     # via
     #   kaya-oidc
     #   kaya-session-redis
     #   tavolo (pyproject.toml)
-kaya-session-redis==0.0.3
+kaya-session-redis==0.0.4
     # via tavolo (pyproject.toml)
 pwo==0.1.2
     # via