zapslog: fix all with slogtest, support inline group, ignore empty group.

[arukiidou]

This change adds a test based on testing/slogtest
that verifies compliance with the slog handler contract
(a draft of this was available in #1335),
and fixes all resulting issues.

The two remaining issues were:

• Group("", attrs) should inline the new fields
  instead of creating a group with an empty name.
  This was fixed with the use of zap.Inline.
• Groups without any attributes should not be created.
  That is, logger.WithGroup("foo").Info("bar") should not
  create an empty "foo" namespace ("foo": {}).
  This was fixed by keeping track of unapplied groups
  and applying them the first time a field is serialized.

Following this change, slogtest passes as expected.

Refs #1333
Resolves #1334, #1401, #1402
Supersedes #1263, #1335

TESTS

• passed. [TEST]zapslog: fix all with slogtest, support inline group, ignore empty group. arukiidou/zap#1
• This also works in Go 1.22

[arukiidou]

@abhinav @tchung1118
Please take a look at this one? Thanks!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (27b96e3) 98.42% compared to head (8466cda) 98.44%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1408      +/-   ##
==========================================
+ Coverage   98.42%   98.44%   +0.01%     
==========================================
  Files          53       53              
  Lines        3495     3527      +32     
==========================================
+ Hits         3440     3472      +32     
  Misses         46       46              
  Partials        9        9              

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[abhinav]

Thanks for this, @arukiidou! Really appreciate the work you put into matching the slog.Handler requirements. This is looking pretty good at a high-level, but there's one issue with the state management: we can't change the handler state like that.
Once a Handler is constructed, consider all its fields immutable. Only new handlers (like the ones returned by WithAttrs) can have different values for those fields—that's the only time we can mutate them. After construction, these handlers will be re-used for concurrent log calls, so they can't modify their own state. I've left suggestions on how I think we could fix the issue.

[abhinav]

Thanks for the quick fix, @arukiidou!

I made a few minor fixups and changed the holdGroup to a slice because the string can't handle cases like WithGroup("foo").WithGroup("bar").Info("msg").

The change looks good to me now. I'll ping a couple of the other maintainers to try to get their approval as well.

Again, thanks for the contribution!

[abhinav]

Got an offline LGTM from @mway.
Merging!

[abhinav]

Thanks, @arukiidou!