metrics: add linear probabilistic counter

[wasaga]

Summary

Adds an utility package with linear probabilistic counter, to be used to estimate session statistics
while only occupying a limited storage space.

Related issues

Related: https://github.com/pomerium/pomerium-zero/issues/821

User Explanation

Checklist

• reference any related issues
• updated docs
• updated unit tests
• updated UPGRADING.md
• add appropriate tag (improvement / bug / etc)
• ready for review

[coveralls]

coverage: 62.786% (+0.05%) from 62.733%
when pulling 2071140 on wasaga/linear-counter
into a246466 on main.

[calebdoxsey]

This is just go.mod changes?

[calebdoxsey]

Code LGTM but I'm not sure what it's for. For counting sessions can't we just do a DB query?

[wasaga]

in order to measure monthly active users you need keep track of all the unique user IDs within a specific period of time (a month, for MAU).

if you just count sessions, say, every hour, that would be a different kind of metric, also useful, but you cannot convert it into a MAU.