counter/countertest: fix %q verb use with wrong type

Caught early by the improved vet check gated behind the 1.26 language
version combined with a tiplang builder that tests with 1.26 language
version.

Fixes golang/go#74262.

Change-Id: I5f5ce82a350b35cebc1224f7d5f8d8a8269c2f6d
Cq-Include-Trybots: luci.golang.try:x_telemetry-gotip-linux-amd64-tiplang
Reviewed-on: https://go-review.googlesource.com/c/telemetry/+/725301
Auto-Submit: Alan Donovan <adonovan@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Commit-Queue: Alan Donovan <adonovan@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

counter/countertest/countertest_test.go

diff --git a/counter/countertest/countertest_test.go b/counter/countertest/countertest_test.go
index 5993230..c6a7150 100644
--- a/counter/countertest/countertest_test.go
+++ b/counter/countertest/countertest_test.go
@@ -47,7 +47,7 @@
 	c := counter.NewStack("foobar", 8)
 
 	if got, err := ReadStackCounter(c); err != nil || len(got) != 0 {
-		t.Errorf("ReadStackCounter = (%q, %v), want (%v, nil)", got, err, 0)
+		t.Errorf("ReadStackCounter = (%#v, %v), want (map with zero elements, nil)", got, err)
 	}
 
 	var wg sync.WaitGroup