dym: make metrics ID inner field public for test/debug

[mathetake]

Commit Message: dym: make metrics ID inner field publish for test/debug
Additional Description:

Otherwise, we cannot mock the logic with the metrics ID without using transmute unsage stuff

Risk Level: n/a
Testing: n/a
Docs Changes: n/a
Release Notes: n/a
Platform Specific Features: n/a

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Caused by: #44115 was opened by mathetake.

see: more, trace.

[akonradi]

Before this change the metric ID values could usefully be used as witness types, so if a EnvoyCounterId existed, the caller could be guaranteed it was valid. Now anybody can construct them. That's useful for testing but makes it easier to write incorrect code.

A pattern I've seen before for this is to have a supertrait that provide associated types for IDs like this. The production type that implements the trait and supertrait would use its real ID types and the test type that implements the trait would provide its own test-only ID types that are easier to construct.

[mathetake]

@akonradi how about providing a constructor only available in #[cfg(test)]?

[akonradi]

That won't work either, unfortunately. #[cfg(test)] only gets set on the crate under test, not on any of its dependencies. The way to do this is to add a Cargo "feature" that enables additional code. That's a common pattern; tokio, for example, has a test-util feature that allows you to advance its mocked clock. We could do something similar here.