Report the size of the guix database and WAL file

Since the Guix daemon doesn't export these metrics, instrument it here.

guix-data-service/utils.scm

@@ -57,6 +57,7 @@
 
             get-gc-metrics-updater
             get-port-metrics-updater
+            get-guix-metrics-updater
 
             call-with-sigint
             run-server/patched
@@ -628,6 +629,26 @@ available.  Return the resource once PROC has returned."
                        (not (member file '("." "..")))))
             '()))))))
 
+(define (get-guix-metrics-updater registry)
+  (define guix-db "/var/guix/db/db.sqlite")
+  (define guix-db-wal (string-append guix-db "-wal"))
+
+  (let ((guix-db-bytes-metric
+         (make-gauge-metric registry "guix_db_bytes"))
+        (guix-db-wal-bytes-metric
+         (make-gauge-metric registry "guix_db_wal_bytes")))
+    (lambda ()
+      (with-exception-handler
+          (lambda _
+            #f)
+        (lambda ()
+          (metric-set guix-db-bytes-metric (stat:size (stat guix-db)))
+          (metric-set guix-db-wal-bytes-metric
+                      (if (file-exists? guix-db-wal)
+                          (stat:size (stat guix-db-wal))
+                          0)))
+        #:unwind? #t))))
+
 ;; This variant of run-server from the fibers library supports running
 ;; multiple servers within one process.
 (define run-server/patched

guix-data-service/web/controller.scm

@@ -201,7 +201,10 @@
           (get-gc-metrics-updater registry))
 
          (port-metrics-updater
-          (get-port-metrics-updater registry)))
+          (get-port-metrics-updater registry))
+
+         (guix-metrics-updater
+          (get-guix-metrics-updater registry)))
 
     (define guile-time-metrics-updater
       (let ((internal-real-time
@@ -437,6 +440,7 @@
 
         (gc-metrics-updater)
         (port-metrics-updater)
+        (guix-metrics-updater)
         (guile-time-metrics-updater)
 
         (list (build-response