Add /.well-known/nodeinfo endpoint

[matrixbot]

This issue was originally created by @6543 at matrix-org/dendrite#3276.

add the nodeinfo endpoint (http://nodeinfo.diaspora.software/)

and add setting to metrics to specify if user statistics should be public or hidden.
if hidden just report the basic information (software, version, ...)

if public, report back total & active users

[matrixbot]

This comment was originally posted by @6543 at matrix-org/dendrite#3276 (comment).

would be used for e.g. https://codeberg.org/thefederationinfo/the-federation.info