Count hits, misses, fetches and transforms as Prometheus counters (closes #311)

The five counts the cache_stats table held are now Prometheus counters in
memory, in internal/imgcache/counters.go, on the default registry that
/metrics serves. The Increment methods add to them, so counting makes no
database call.

cache_stats, Cache.Stats, Service.Stats, CacheStats and the pending counts
are gone, with their tests. The background goroutine now writes only access
times, and its names say so: StartPendingAccessTimeWrites,
StopPendingAccessTimeWrites and pendingaccesstimes.go.

The counters count for the whole process, so the tests that check how much
they rose do not call t.Parallel. Two stop tests whose write under way was a
hit count now use an access time.

Model: opus-5-5

internal/imgcache/concurrent_misses_internal_test.go

@@ -143,13 +143,15 @@ func jpegSize(data []byte) (int, int) {
 // one uncached variant while the first one's fetch is held. Between them they
 // must fetch the source once and transcode it once, every one must be answered
 // with the same 50x25 JPEG, and each must count one miss.
+//
+//nolint:paralleltest // it reads counters that other tests add to
 func TestService_Get_ConcurrentMissesShareOneFetch(t *testing.T) {
-	t.Parallel()
-
 	svc, fixtures, fetcher := setupHeldFetchService(t)
 
 	const requests = 8
 
+	before := readCounters()
+
 	pending := make([]<-chan getResult, 0, requests)
 	for range requests {
 		pending = append(pending,
@@ -191,12 +193,10 @@ func TestService_Get_ConcurrentMissesShareOneFetch(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	want := cacheStatsCounters{0, requests, 1, int64(len(photo)), 1}
+	want := counterValues{0, requests, 1, int64(len(photo)), 1}
 
-	waitForPendingCountsWritten(t, svc.cache)
-
-	if got := readCacheStatsCounters(t, svc.cache); got != want {
-		t.Errorf("counters = %+v, want %+v", got, want)
+	if got := readCounters().minus(before); got != want {
+		t.Errorf("the requests added %+v to the counters, want %+v", got, want)
 	}
 }