gopls/internal/test/integration: reduce flakes in TestTelemetryPrompt

Issue golang/go#71590 describes the likely source of flakiness of
TestTelemetryPrompt_Response: the test will flake if the UTC day has
changed between the start of TestMain and the start of the test. While
we cannot fix this flakiness without a change to x/telemetry, we can
significantly reduce it by first causing a file rotation check at the
start of the test.

For golang/go#68659

Change-Id: I22b6b41ebadbea1f4af0b7d4dc64dbfcf617cefd
Reviewed-on: https://go-review.googlesource.com/c/tools/+/647436
Auto-Submit: Robert Findley <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Jonathan Amsterdam <[email protected]>

Author: findleyr

gopls/internal/test/integration/misc/prompt_test.go

@@ -276,6 +276,19 @@ func main() {
 		allCounters       = []string{acceptanceCounter, declinedCounter, attempt1Counter}
 	)
 
+	// To avoid (but not prevent) the flakes encountered in golang/go#68659, we
+	// need to perform our first read before starting to increment counters.
+	//
+	// ReadCounter checks to see if the counter file needs to be rotated before
+	// reading. When files are rotated, all previous counts are lost. Calling
+	// ReadCounter here reduces the window for a flake due to this rotation (the
+	// file was originally was located during countertest.Open in TestMain).
+	//
+	// golang/go#71590 tracks the larger problems with the countertest library.
+	//
+	// (The counter name below is arbitrary.)
+	_, _ = countertest.ReadCounter(counter.New("issue68659"))
+
 	// We must increment counters in order for the initial reads below to
 	// succeed.
 	//