Warn about skipping reactor metrics when using unknown reactor type (#19383)

Spawning from not seeing any reactor metrics in the Grafana dashboard in
some load tests, noticing `python_twisted_reactor_tick_time_bucket` is
`0` in Prometheus, following it back to Synapse and seeing that we don't
warn about skipping reactor metrics in all cases (when using an unknown
reactor type).

A follow-up to this would be to actually figure out how to instrument
the `ProxiedReactor` or why `ProxiedReactor` is being chosen in the
first place and see if we can get it to use a more normal type
🤔


### Reproduction instructions

1. Using the Complement scripts **with workers**: `WORKERS=1
./scripts-dev/complement.sh ./tests/csapi`
 1. `docker logs complement_csapi_dirty_hs1 2>&1 | grep -i "reactor"`
1. With these changes, notice `Skipping configuring
ReactorLastSeenMetric: unexpected reactor type: <__main__.ProxiedReactor
object at 0x7fc0adaaea50>` and `Twisted reactor: ProxiedReactor`
 1. Cleanup:
- `docker stop $(docker ps --all --filter "label=complement_context"
--quiet)`
- `docker rm $(docker ps --all --filter "label=complement_context"
--quiet)`

I'm unable to reproduce with the normal Synapse images or
`complement-synapse` without workers. They all use `Twisted reactor:
EPollReactor`

<details>
<summary>Checking <code>docker/Dockerfile-workers</code></summary>

1. Build the Docker image for Synapse: `docker build -t
matrixdotorg/synapse -f docker/Dockerfile . && docker build -t
matrixdotorg/synapse-workers -f docker/Dockerfile-workers .`
([docs](7a24fafbc3/docker/README-testing.md (building-and-running-the-images-manually)))
 1. Start Synapse:
     ```
    docker run -d --name synapse \
        --mount type=volume,src=synapse-data,dst=/data \
        -e SYNAPSE_SERVER_NAME=my.docker.synapse.server \
        -e SYNAPSE_REPORT_STATS=no \
        -e SYNAPSE_ENABLE_METRICS=1 \
        -p 8008:8008 \
        -p 9469:9469 \
        matrixdotorg/synapse-workers:latest
    ```
 1. `docker logs synapse 2>&1 | grep -i "reactor"`
 1. Says `Twisted reactor: EPollReactor`
 
 </details>

changelog.d/19383.misc

@@ -0,0 +1 @@
+Warn about skipping reactor metrics when using unknown reactor type.

synapse/metrics/_reactor_metrics.py

@@ -158,12 +158,17 @@ try:
         else:
             # E.g. this does not support the (Windows-only) ProactorEventLoop.
             logger.warning(
-                "Skipping configuring ReactorLastSeenMetric: unexpected asyncio loop selector: %r via %r",
+                "Skipping configuring reactor metrics: unexpected asyncio loop selector: %r via %r",
                 selector,
                 asyncio_loop,
             )
+    else:
+        logger.warning(
+            "Skipping configuring reactor metrics: unexpected reactor type: %r",
+            reactor,
+        )
 except Exception as e:
-    logger.warning("Configuring ReactorLastSeenMetric failed: %r", e)
+    logger.warning("Configuring reactor metrics failed: %r", e)
 
 
 if wrapper: