appview/metrics: add prometheus HTTP metrics, middleware, and endpoint

The default metrics listener port is 127.0.0.1:7101, so it should only be accessible from the host itself, not the web. Port 7000 is where knotmirror currently exports, so 7101 is chosen to not collide. Lastly, this incidentally exposes lower-level go client metrics (e.g., garbage collection info) as well as AT and Jetstream metrics already calculated by the bsky indigo and jetstream packages - which could be very useful. Signed-off-by: Kevin <tangled-jj@paeth.xyz>