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/handlers/cache_hit_internal_test.go

@@ -101,9 +101,8 @@ func newCountingDatabase(tb testing.TB) (*sql.DB, *atomic.Int64) {
 // newCountingDatabaseServer returns a router for both image routes whose
 // image cache uses newCountingDatabase, the count of the statements run on
 // that database, and a URL on each route for a different image of
-// photoPath on countingHost. Nothing writes the cache's pending counts and
-// access times in the background, so every statement counted comes from a
-// request.
+// photoPath on countingHost. Nothing writes the cache's access times in the
+// background, so every statement counted comes from a request.
 func newCountingDatabaseServer(
 	tb testing.TB,
 ) (http.Handler, *atomic.Int64, []string) {