Fix race when reporting internal cardinality metrics

[vdarulis]

Previous attempt to fix it was incomplete - added a test that reliably triggers this.
Note that there was no need to track interm counts atomically, the subscope iterator is executed serially.

==================
WARNING: DATA RACE
Write at 0x00c000194588 by goroutine 8:
  github.com/uber-go/tally/v4.(*scope).Gauge()
      /Users/vytenis/go/tally/scope.go:341 +0x3a4
  github.com/uber-go/tally/v4.TestCachedReporterInternalMetricsConcurrent.func2()
      /Users/vytenis/go/tally/scope_registry_test.go:293 +0x114

Previous read at 0x00c000194588 by goroutine 9:
  github.com/uber-go/tally/v4.(*scopeRegistry).reportInternalMetrics.func1()
      /Users/vytenis/go/tally/scope_registry.go:314 +0xe0
  github.com/uber-go/tally/v4.(*scopeRegistry).ForEachScope()
      /Users/vytenis/go/tally/scope_registry.go:159 +0x118
  github.com/uber-go/tally/v4.(*scopeRegistry).reportInternalMetrics()
      /Users/vytenis/go/tally/scope_registry.go:310 +0x1e4
  github.com/uber-go/tally/v4.(*scopeRegistry).CachedReport()
      /Users/vytenis/go/tally/scope_registry.go:137 +0x70
  github.com/uber-go/tally/v4.(*scope).reportRegistry()
      /Users/vytenis/go/tally/scope.go:278 +0xc4
  github.com/uber-go/tally/v4.(*scope).reportLoopRun()
      /Users/vytenis/go/tally/scope.go:270 +0x58
  github.com/uber-go/tally/v4.TestCachedReporterInternalMetricsConcurrent.func3()
      /Users/vytenis/go/tally/scope_registry_test.go:316 +0x134

Goroutine 8 (running) created at:
  github.com/uber-go/tally/v4.TestCachedReporterInternalMetricsConcurrent()
      /Users/vytenis/go/tally/scope_registry_test.go:280 +0x3cc
  testing.tRunner()
      /opt/homebrew/Cellar/[email protected]/1.21.9/libexec/src/testing/testing.go:1595 +0x1b0
  testing.(*T).Run.func1()
      /opt/homebrew/Cellar/[email protected]/1.21.9/libexec/src/testing/testing.go:1648 +0x40

Goroutine 9 (running) created at:
  github.com/uber-go/tally/v4.TestCachedReporterInternalMetricsConcurrent()
      /Users/vytenis/go/tally/scope_registry_test.go:302 +0x4f0
  testing.tRunner()
      /opt/homebrew/Cellar/[email protected]/1.21.9/libexec/src/testing/testing.go:1595 +0x1b0
  testing.(*T).Run.func1()
      /opt/homebrew/Cellar/[email protected]/1.21.9/libexec/src/testing/testing.go:1648 +0x40
==================

[abliqo]

Nice, thank you!