Automatically serve /metrics on port 9464

[brettimus]

From the Typescript docs:

By default the TypeScript library makes the metrics available on <your_host>:9464/metrics. Make sure your Prometheus is configured correctly to find it.

autometrics-py should do something similar!

[actualwitch]

i think there's still no decision on how to standardize it
https://github.com/orgs/autometrics-dev/discussions/32

[IvanMerrill]

if we don't do this then we should update the python documentation to show that the metrics endpoint needs to be created. This is what we've done in the SLO guides, and it's done in a couple of different ways in the examples (either specifically creating the endpoint or using the prometheus client to start an http server) but it's not in the basic documentation.