fix: go-kit/log-slog adapter fixes

[tjhop]

Changes included:

• log/slog always includes timestamps by default; they are now included in the go-kit/log output. We make no attempts to adjust/remove any other timestamp keys (such as the often used ts kv pair in go-kit/log implementations).
• a new function to handle properly appending/nesting our kv pairs for the logger
• proper handling of nested grouping
• introduction of a preformatted field on the goKitHandler struct to cache previously added kv pairs containing log values. helpful to maintain the ordering of kv pairs added via multiple/nested/chained calls to a logger's .With() or .WithGroup() methods.
• WithGroup() now returns a new handler struct, as opposed to updating itself (as recommended by slog implementation docs).

Not addressed:

• tests still need to be updated. they currently use a mock logger, and the log lines have timestamps included. Since timestamps aren't deterministic, they're not easy to mock. I refrained from creating a wrapper interface to mock the time just for sake of testing; I used pattern matching with regexs when implementing slog tests for prometheus/common's promslog pkg.
• I haven't found a good way to (programmatically) address call depth when getting source caller info. Using a call depth of 6 (see code snippet) seems to properly unwind back to the originating .Info() etc method call, but I haven't checked other options.

Example output, after shimming the following logging calls into the existing handler test suite:

gkadapterLogger := SlogFromGoKit(log.With(log.NewLogfmtLogger(os.Stderr), "caller", log.Caller(6)))
gkadapterLogger.Info("go-kit/log adapter output")
gkadapterLogger.WithGroup("g1").WithGroup("g2").Info("test1", "hello", "world")
gkadapterLogger.WithGroup("g1").With("hello", "world").Info("test2", "hello", "world")
gkadapterLogger.WithGroup("g1").With("hello", "world").WithGroup("g2").Info("test3", "hello", "world")
gkadapterLogger.With("hello", "world").With("hello", "world").Info("test4", "hello", "world")
// Output:
// level=info caller=slogadapter_test.go:26 time=2024-11-18T13:09:59.674350655-05:00 msg=test1 g1.g2.hello=world
// level=info caller=slogadapter_test.go:27 time=2024-11-18T13:09:59.674376383-05:00 msg=test2 g1.hello=world g1.hello=world
// level=info caller=slogadapter_test.go:28 time=2024-11-18T13:09:59.674392598-05:00 msg=test3 g1.hello=world g1.g2.hello=world
// level=info caller=slogadapter_test.go:29 time=2024-11-18T13:09:59.674419421-05:00 msg=test4 hello=world hello=world hello=world

What this PR does

Which issue(s) this PR fixes or relates to

Fixes #

Checklist

• Tests updated.
• Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX].
• about-versioning.md updated with experimental features.

[CLAassistant]

All committers have signed the CLA.

[aknuds1]

LGTM, thanks

Tests are failing.

[aknuds1]

Can you fix failing tests?