feat: Log key prefix controls for Trainer, WandbLogger, and CometLogger

[pupeno]

What does this PR do?

Fixes #21782

Adds three independent parameters that let users control the metric keys generated by Lightning's
own components, without touching user-provided metric names:

• Trainer(log_key_prefix=...): prefixes Trainer-generated metric keys. Currently the only such
  key is the automatic epoch metric injected into every logging step. Default None/""
  preserves previous behaviour.

• WandbLogger(log_key_prefix=...): controls the W&B-generated step key used in define_metric
  and experiment.log. Previously hardcoded to "trainer/global_step"; now configurable,
  defaulting to "global_step". The existing prefix= argument is unchanged and continues to
  apply only to pass-through metric keys.

• CometLogger(epoch_key=...): controls which incoming metric key is extracted and forwarded to
  Comet's dedicated epoch= argument. Previously hardcoded to "epoch". None disables epoch
  extraction entirely. Default "epoch" preserves previous behaviour.

The design principle: each log_key_prefix (or epoch_key) applies only to keys generated by
that component, never to keys the user logged via self.log(...) or logger.log_metrics(...).

This makes it straightforward to align all Lightning-generated keys under a shared namespace:

Trainer(log_key_prefix="trainer/")
WandbLogger(log_key_prefix="trainer/")
CometLogger(epoch_key="trainer/epoch")

Breaking change: WandbLogger previously hardcoded "trainer/global_step" as the step axis
key. With this PR, the default is bare "global_step". Pass log_key_prefix="trainer/" to
restore the previous behaviour.

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[deependujha]

Thanks for the thorough proposal and PR, the namespace inconsistency you're describing is real, and the Trainer-side piece is clean.

Would you be open to splitting this into two PRs? I think it'll move faster:

PR 1: Trainer only. The log_key_prefix kwarg on Trainer plus the one-line change in logger_connector.py (setdefault(f"{prefix}epoch", ...)). This part is fully additive and backward-compatible — defaults to bare epoch, so nothing changes for existing users. And because the key is injected upstream in the logger connector before any logger sees it, this benefits every logger downstream without touching any logger file. That makes it reviewable on the Trainer side independently.

PR 2: per-logger keys. Each logger's handling of the keys it injects itself. Those touch logger files and need the respective codeowners, so keeping them separate avoids blocking the Trainer change on that review.

If you split it, the Trainer PR should be straightforward to land. Thanks again.

[pupeno]

Ah, yes, that makes sense. I'll have it in a moment. Do you want me to create separate github issues or only separate PRs?

[pupeno]

Here are the PRs. I can also create the github issues if you want:

• feat: add log_key_prefix to Trainer for Trainer-generated metric keys #21784
• feat: add log_key_prefix to WandbLogger; default global_step key drops trainer/ prefix #21785
• feat: add epoch_key to CometLogger to make epoch extraction configurable #21786

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✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79%. Comparing base (fe6b1cc) to head (eb9bfda).
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lightning_fabric	630	0
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pytorch2.5.1	72	3
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pytorch2.3	70	3

Additional details and impacted files

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- Misses       3229     5125   +1896