feat(tvix): add instance_name to instrumentation of *Services

Currently it is not possible to distinguish between tracing of the same
*Service type whenever there are multiple of them. Now the instance_name
of ServiceBuilder is passed into the *Service and used in the existing
instrument as the `instance_name` field.

Places that did not already have a instance_name in its context use
`"default"`. In tests I used `"test"`.

Change-Id: Ia20bf2a7bb849a781e370d087ba7ddb3be79f654
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12739
Tested-by: BuildkiteCI
Autosubmit: Bob van der Linden <bobvanderlinden@gmail.com>
Reviewed-by: flokli <flokli@flokli.de>

tvix/docs/src/TODO.md

@@ -189,10 +189,6 @@ This requires some more designing. Some goals:
      - Maybe add a ?cache=$other_url parameter support to the URL syntax, to
        easily wrap a store with a caching frontend, using $other_url as the
       "near" store URL.
-    - Each store should get its instance name passed down, and add this as a
-      field in the instrumentation calls. This causes log messages and
-      per-instance store metrics to be traceable back to the specific instance
-      (if multiple backends of the same type are present).
 
 ### Store Config
    There's already serde for some store options (bigtable uses `serde_qs`).