Consider enforcing no leading underscore in tag keys

I would like to consider ensuring that the storage engine does not accept any tag keys with leading underscores, essentially leaving the `_` prefix as a private key space for internal series key information.

With that as an invariant, we could make certain performance improvements around reading and parsing keys.

The issue names no files, tests, or entry points. Start by locating storage-engine tag-key validation and the code that reads or parses series keys. Define the invariant and its compatibility implications before identifying tests that show user tag keys with leading underscores are rejected while internal series-key information remains supported.

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