Add useWinstonMetaAsLabels option

[MasterIO02]

When enabled, this option pushes the "rest" of Winston's log info to Loki labels.

This is useful to pass dynamic labels to Loki from the Winston log method directly, for example APIs HTTP methods or status codes.

This PR seems to contradict PR #103 that is still open, but since then, Grafana Loki devs are using dynamic labels in their docs for storing status codes or HTTP methods so it seems fair to use Winston's metadata to pass to Loki's labels, as long as they're not totally dynamic (e.g. different IDs for every log).

This option is disabled by default and overrides the "labels" option, because imo the fixed labels should be defined in Winston directly with the "defaultMeta" option when initializing a new Logger, but other ways might be better.

I named the option useWinstonMetaAsLabels because Winston considers anything other than "level" and "message" to be "meta" in their docs, though it could be shorter.

It should probably be paired with some sort of whitelist/blacklist to be able to only use some Winston meta as Loki labels, but I'm not sure which is best to implement.

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[MasterIO02]

After some reflexion, I've added an "ignoredMeta" option that will make the useWinstonMetaAsLabels option ignore the meta tags provided in that array.

This is useful when we want to have Winston handle the logging of non-string values (like errors) to the console, but not have them as labels in Loki.

[JaniAnttonen]

Thanks for this succinct and well described PR! I'll draft a new release from this ASAP, and probably will simplify the config further defaulting on this logic in a future release.