gRPC logging/tracing middleware

[q3k]

We currently have no way to log anything in the Curator's gRPC handlers. This isn't great.

We should use the opportunity to introduce this properly, eg. integrate with some tracing system, or at least make it possible to easily integrate with such a tracing system later.

Some design decisions to be made:

• Do we make supervisor.Logger(ctx) just work? How supervisor-agnostic do we want to make this? We likely want to use logtree interfaces, but maybe not refer to the supervisor package unless necessary? Is the clean separation worth having to do supervisor.Logger in runnables vs. somethingelse.Logger in RPCs?
• Where do we actually log things from requests? Somewhere within the logtree of the supervisor runnable, or to a totally separate set of buffers?

[q3k]

(Not working on this right now)

[q3k]

Working on a design for this now.

[q3k]

Work in progress merging some prerequisites:

https://review.monogon.dev/c/monogon/+/536

https://review.monogon.dev/c/monogon/+/538

[q3k]

The design will start off with a thing wrapper that implements an OpenTelemetry-like Trace/Span API directly backed by logtree, only supporting emitting events.

Then we fully implement OpenTelemetry support with proper local/remote spans and emitting spans to collectors, tying spans into other components, etc.

[q3k]

Changes implementing tracing/logging out now:

https://review.monogon.dev/c/monogon/+/541

https://review.monogon.dev/c/monogon/+/542

[q3k]

Done. Filing a follow up non-MVP issue about 'full' tracing.

#106