Wrapper for slog

[leonorfmartins]

What is the chore?:
Problem 1: there are different log packages in the grafana codebase, we should just stick to one.
Problem 2: direct dependencies on pkg/infra/log (which in its turn has plenty of dependencies) makes it difficult to decompose the backend into independent modules.

Proposal:

pkg/infra/log is still useful (and we should not make incompatible changes to the logger output). It handles various per-service logging levels, supports syslog and colourised logs and a few other things. Its interface is fairly standard (4 levels + key value attributes + context).

However, Go now comes with slog in its standard library. We should be able to convert existing pkg/infra/log.Logger into a Handler (https://pkg.go.dev/log/slog#Handler) for slog. We could also inject a Handler through Wire and/or make global slog logger to use our custom Handler. This would allow services to use slog APIs for logging without worrying what kind of a logging backend they are using.

As a positive side effect it also breaks the dependency graph and loosens the connections to pkg/infra.

(We could do the same thing for traces and use OTEL interface everywhere as well) - however here I'd suggest to focus only on logging topic.

Related PR: #87000

Explore

Beta Give feedback

1. Adapter from infra/log.Logger to slog.Handler
2. Provide slog.Handler into Wire
3. Helper for FromContext logger
4. Can pkg/infra/log replace default slog logger with the one with a custom Handler? NOT YET! Depends on the order of Wire initialisation
5. Is the output still compatible between slog and pkg/infra/log? Yes
6. Is it possible to find/replace all log.Logger calls and convert them to slog API? (yes, but need to differentiate between Context and non-Context calls somehow)
7. Which slog.Handler to use for tests? (provided a nop logger and a wrapper for t.Log)
8. Fix: control per-service logger levels (now it seems to run on start, should be more dynamic)

Implement

Beta Give feedback

1. A script to rename existing log with glog (and remove it later)
2. A script to replace logger constructor with slog
3. A script to replace logger Info/Debug/Warn/Error methods
4. A script to detect if a method has a context and a custom logger FromContext: then replace it with service logger and Context methods

Finish

Beta Give feedback

1. Can we tweak the initialisation process to set Grafana's pkg/infra/log Handler as the default, so that the clients could use slog.Default().With("logger", "mylogger") without any other imports?

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