gopls/internal/telemetry/cmd/stacks: add dry run flag

This flag avoids updating existing issues. It still requires a GitHub
auth token to determine which issues would get updated. It also still
opens a browser window for new issues.

For golang/go#71045.

Change-Id: I6a6a636c26a402c9ea66160e14cd388b490b74b9
Reviewed-on: https://go-review.googlesource.com/c/tools/+/642421
Auto-Submit: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

gopls/internal/telemetry/cmd/stacks/stacks.go

@@ -94,6 +94,8 @@ var (
 
 	daysFlag = flag.Int("days", 7, "number of previous days of telemetry data to read")
 
+	dryRun = flag.Bool("n", false, "dry run, avoid updating issues")
+
 	authToken string // mandatory GitHub authentication token (for R/W issues access)
 )
 
@@ -559,6 +561,12 @@ func updateIssues(issues []*Issue, stacks map[string]map[Info]int64, stackToURL
 			newStackIDs = append(newStackIDs, id)
 			writeStackComment(comment, stack, id, stackToURL[stack], stacks[stack])
 		}
+
+		if *dryRun {
+			log.Printf("DRY RUN: would add stacks %s to issue #%d", newStackIDs, issue.Number)
+			continue
+		}
+
 		if err := addIssueComment(issue.Number, comment.String()); err != nil {
 			log.Println(err)
 			continue