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

README.md

@@ -88,14 +88,18 @@ A load balancer's health check can request `/.well-known/healthcheck.json`,
 which answers 200 whenever pixa is running, in maintenance mode too (see
 `maintenance_mode`).
 
-On SIGTERM or SIGINT pixa stops accepting connections, gives the requests in
-progress and the images being processed 5 seconds to finish, then writes to the
-database the counts of cache hits, misses, fetches and conversions and the times
-cached images were last used that are not written yet (see Storage), and exits:
-with 0, or with 1 when images were still being processed after those 5 seconds,
-some of those counts or times could not be written, or another part of pixa
-failed to stop. A request not finished by then is cut off. `docker stop` waits
-10 seconds before it kills the container.
+On SIGTERM or SIGINT pixa stops accepting connections. The requests in progress
+and the images being processed get 5 seconds to finish. A request not finished
+by then is cut off. pixa then writes to the database the times cached images
+were last used that are not written yet (see Storage), and exits. It exits with
+1 when:
+
+- images were still being processed after those 5 seconds,
+- some of those times could not be written, or
+- another part of pixa failed to stop.
+
+Otherwise it exits with 0. `docker stop` waits 10 seconds before it kills the
+container.
 
 Outside Docker, pixa needs libvips (the image has 8.16) and libheif to run, as
 it uses libvips through CGO. pixad does not start unless libvips has its JPEG XL
@@ -209,14 +213,14 @@ Test). Toward it, a cache hit opens the transformed image's file and nothing
 else, and makes no database call. The content type comes from the file's first
 bytes, which are those of the format pixa wrote it in; a file whose first bytes
 are not those of a format pixa writes is not served, and the image is converted
-again. The time the image was last served, which eviction orders by, and the hit
-count are handed to a background writer without waiting for it, as are the
-counts of misses, fetches and conversions, and the time a miss last used a
-cached source image. Once a second that writer writes the waiting times in one
-transaction and the counts in one statement. While 10,000 times of transformed
-images, or 10,000 of source images, wait to be written, a further one is
-dropped, and that image only looks less recently used to eviction.
-`script/bench` measures how many hits pixa serves per second (see Load Test).
+again. The time the image was last served, which eviction orders by, is handed
+to a background writer without waiting for it. The hit is counted in memory. A
+miss also hands that writer the time it last used a cached source image. Once a
+second that writer writes the waiting times in one transaction. While 10,000
+times of transformed images, or 10,000 of source images, wait to be written, a
+further one is dropped, and that image only looks less recently used to
+eviction. `script/bench` measures how many hits pixa serves per second (see Load
+Test).
 
 ### Routes
 
@@ -814,15 +818,24 @@ See `configs/config.example.yml` for all options with defaults.
 - **Image processing**: govips (CGO wrapper for libvips)
 - **Database**: SQLite via modernc.org/sqlite
 - **Static assets**: embedded via `//go:embed`
-- **Metrics**: Prometheus, at `/metrics`: generic HTTP request metrics
-  (duration, response size, requests in flight) and the Go runtime and process
-  metrics; requests are measured and `/metrics` is served only when
-  `metrics.username` and `metrics.password` are set. A request's `handler` label
-  is the pattern of the route it matches, such as `/v1/image/*`, never its path.
-  Requests that match no route share the label `unmatched`: those the router
-  answers 404 or 405, and CORS preflights. A request's `method` label is its
-  method when that is `GET`, `HEAD`, `POST`, `PUT`, `PATCH`, `DELETE`,
-  `OPTIONS`, `CONNECT` or `TRACE`, and `other` for any other method
+- **Metrics**: Prometheus, at `/metrics`. pixa serves `/metrics` and measures
+  HTTP requests only when `metrics.username` and `metrics.password` are set. A
+  request's `handler` label is the pattern of the route it matches, such as
+  `/v1/image/*`, never its path. Requests that match no route share the label
+  `unmatched`: those the router answers 404 or 405, and CORS preflights. A
+  request's `method` label is its method when that is `GET`, `HEAD`, `POST`,
+  `PUT`, `PATCH`, `DELETE`, `OPTIONS`, `CONNECT` or `TRACE`, and `other` for any
+  other method. `/metrics` holds:
+    - the duration of HTTP requests
+    - the size of HTTP responses
+    - the HTTP requests in flight
+    - the Go runtime and process metrics
+    - `pixa_cache_hits_total`: image requests served from the cache
+    - `pixa_cache_misses_total`: image requests not served from the cache
+    - `pixa_upstream_fetches_total`: fetches of a source image that read any
+      bytes
+    - `pixa_upstream_fetch_bytes_total`: the bytes those fetches read
+    - `pixa_transforms_total`: images converted
 - **Logging**: stdlib slog
 
 ## Entrypoints