Rageshakes should include the HS version

[jryans]

It would be helpful to include the homeserver version in the rageshake metadata.

I suppose the Server header on responses is not a reliable indicator (as it can be masked by a CDN), but maybe still worth including?

We could also request _synapse/admin/v1/server_version from the HS, but that's obviously Synapse-specific.

[t3chguy]

We could also request _synapse/admin/v1/server_version from the HS, but that's obviously Synapse-specific.

And not exposed on matrix.org as it does not forward /_synapse/ reqs beyond the HAProxy

[turt2live]

Would be good for a server info endpoint, if we ever pick one of the 3 MSCs...