Tag trace with instance name (#13761)

We tag the Synapse instance name so that it's an easy jumping off point into the logs. Can also be used to filter for an instance that is under load.

As suggested by @clokep and @reivilibre in,

 - https://github.com/matrix-org/synapse/pull/13729#discussion_r964719258
 - https://github.com/matrix-org/synapse/pull/13729#discussion_r964733578