Rewrite Prometheus exporter tests

[pellared]

Fixes #4270

• Rewrite TesInvalidInsrtrumentForPrometheusIsIgnored into TestIncompatibleMeterName - it was not being executed (sic!) because of a bad name (my fault).
  The test should be more readable and it uses exported API.
  It is also more stable and does not uses goroutines which were not needed.
  The test is based on TestPrometheusExporter.
  I have also created Meter name validation opentelemetry-specification#3579

• Rewrite TestCollectConcurrentSafe into TestCollectorConcurrentSafe.
  I reimplemented the concurrency test so that it uses exported API.
  It should not leak any goroutines (as previously the goroutines were leaking).
  I double-checked that exporter.Collect is indeed called concurrently (I used this hack to triple-check):

$ go test -timeout 30s -run ^TestCollectorConcurrentSafe$ -race
Collect start: 50
Collect start: 35
Collect end: 50
Collect start: 5
Collect end: 35
Collect start: 52
Collect start: 32
Collect end: 32
Collect end: 52
Collect end: 5
Collect start: 37
Collect start: 54
Collect start: 66
Collect start: 7
Collect start: 68
Collect end: 37
Collect end: 54
Collect end: 66
Collect end: 7
Collect end: 68
PASS
ok      go.opentelemetry.io/otel/exporters/prometheus   0.032s

[pellared]

FYI @hexdigest

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.5%. Comparing base (10c3445) to head (cbd8a87).
Report is 1566 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #4274   +/-   ##
=====================================
  Coverage   83.5%   83.5%           
=====================================
  Files        183     183           
  Lines      14207   14207           
=====================================
+ Hits       11869   11871    +2     
  Misses      2110    2110           
+ Partials     228     226    -2     

see 2 files with indirect coverage changes

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