fix: record previous value of sampler counter metrics so they report correctly

[kentquirk]

Which problem is this PR solving?

• Fixes Counter-type metrics for samplers are broken #1279

Short description of the changes

• Update the previous values
• Fix a test that was sensitive to existing standard envvar

[VinozzZ]

Looks good to me. Is there a way to add test for the sampler to make sure we are not going to break it in the future?

[kentquirk]

Well, I tried, but it ended up being an extremely picky, implementation-dependent test -- not only on the implementation of the sampler, but also the implementations of metrics AND MockMetrics. I didn't think it was worth it, so I left it off.