Move global random number generator to randRes field

[MrAlias]

Instead of using a global random number generator for all randRes, have each value use its own. This removes the need for locking and managing concurrent safe access to the global. Also, the field, given the Reservoir type is not concurrent safe and the metric pipeline guards this, does not need a sync.Mutex to guard it.

Supersedes #5815
Fix #5814

Performance Analysis

This change has approximately equivalent performance as the existing code based on existing benchmarks.

goos: linux
goarch: amd64
pkg: go.opentelemetry.io/otel/sdk/metric
cpu: Intel(R) Core(TM) i7-8550U CPU @ 1.80GHz
                           │   old.txt   │              new.txt               │
                           │   sec/op    │   sec/op     vs base               │
Exemplars/Int64Counter/8-8   14.00µ ± 3%   13.44µ ± 4%  -3.98% (p=0.001 n=10)

                           │   old.txt    │             new.txt              │
                           │     B/op     │     B/op      vs base            │
Exemplars/Int64Counter/8-8   3.791Ki ± 0%   3.791Ki ± 0%  ~ (p=1.000 n=10) ¹
¹ all samples are equal

                           │  old.txt   │            new.txt             │
                           │ allocs/op  │ allocs/op   vs base            │
Exemplars/Int64Counter/8-8   84.00 ± 0%   84.00 ± 0%  ~ (p=1.000 n=10) ¹
¹ all samples are equal

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.5%. Comparing base (a7e83aa) to head (9f4e8cd).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #5819   +/-   ##
=====================================
  Coverage   84.5%   84.5%           
=====================================
  Files        272     272           
  Lines      22756   22758    +2     
=====================================
+ Hits       19248   19252    +4     
+ Misses      3165    3163    -2     
  Partials     343     343           

see 2 files with indirect coverage changes