internal: true up internal metrics collection for post-SRA middleware

[lucix-aws]

• correct timings for computation of endpoint resolution, signing, and identity retrieval metrics
• remove old timings from deprecated sigv4 middleware (not strictly necessary but it removes the package dependency)
• some of these files appear to have not been touched by gofmt previously - there are some incidental changes therein

[Madrigal]

Should this be GetIdentityStartTime to follow the other MetricData members convention?

[lucix-aws]

I elected to not since I felt it was obvious in the types, but I can change it. Note this API (all of this) is behind a private/"don't use, will break" disclaimer as well.

[Madrigal]

I'd go with consistency rather than make it obvious with the types

[Madrigal]

I need a bit more context as to why are we removing these

[lucix-aws]

It's no longer a per-attempt metric with the SRA auth refactor - credential (abstractly referred to now as identity) retrieval happened as part of the signing process in the now-deprecated sigv4 middleware, as part of an attempt lifecycle. Now it's its own middleware (GetIdentity) separate from signing and before the retry middleware i.e. before the attempt loop starts.

Long-term I think we do want to pull the start of the retry loop further back but that's out of scope for now.