The problem with many services is that it makes it hard to find which service has the trace you want, see https://github.com/jaegertracing/jaeger-ui/issues/985

Previously, we split traces out into services based on their instance name like `matrix.org client_reader-1`, etc but there are many worker instances of the same `client_reader` so there is a lot to click through.

With this PR, all of the traces are just collected under the worker type like `client_reader`, `event_persister` 😇

Note: A Synapse worker instance name is an opaque string with the number convention only being our own thing for the `matrix.org` deployment. But seems pretty sensible to group things this way.