internal/counter: use ReadMapped from counter.Read

counter.Read, which is the implementation of countertest.ReadCounter,
was using os.ReadFile to read the counter file's contents. As we have
seen elsewhere, this may lead to synchronization issues. To ensure we
observe all active counts we should read via mmap. For this reason,
counter.ReadFile was already using the ReadMapped helper. Update Read to
do the same. I believe this fixes two test failures that we've observed
in gopls.

For golang/go#68659
For golang/go#69132

Change-Id: I8d09b98fa2e69ac1b9a7580b061d4b266130773c
Reviewed-on: https://go-review.googlesource.com/c/telemetry/+/625496
Auto-Submit: Robert Findley <rfindley@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: findleyr

internal/counter/counter.go

@@ -341,7 +341,7 @@ func readFile(f *file) (*File, error) {
 		return nil, fmt.Errorf("counter has no mapped file")
 	}
 	name := current.f.Name()
-	data, err := os.ReadFile(name)
+	data, err := ReadMapped(name)
 	if err != nil {
 		return nil, fmt.Errorf("failed to read from file: %v", err)
 	}