Move tracing-subscriber test/support module into tracing-mock

[hds]

Feature Request

Refactor tracing-subscriber::tests::support and tracing-mock so that some items currently defined as public in tracing-mock can be made not-public (specifically pub(crate)).

Crates

• tracing-subscriber
• tracing-mock

Motivation

While documenting tracing-mock, I found a couple of things in the collector module which are public, but don't need to be except that they're used in the module tracing-subscriber::tests::support.

In the context of #539, this is potentially exposing parts of tracing-mock that it would be better not to expose. For example the num collector::Expect is currenlty public, leaving it this way would make adding new expectations a breaking change.

Proposal

Move everything needed by tracing-subscriber tests (that uses potential tracing-mock internals) into tracing-mock itself and put it behind a feature flag.

Alternatives

Another solution would be to duplicate the required parts of tracing-mock in the support module - which I think is how it was set up before.

Or these public items could be reworked to ensure forward comaptibility, for example by adding #[non_exhaustive] to the previously mentioned Expect enum.