Expose Guile GC metrics

As the guix-data-service process seems to be using excessive amounts of
memory, and this will be useful to track it.

guix-data-service/utils.scm

@@ -24,6 +24,7 @@
   #:use-module (fibers)
   #:use-module (fibers channels)
   #:use-module (fibers conditions)
+  #:use-module (prometheus)
   #:export (call-with-time-logging
             with-time-logging
             prevent-inlining-for-tests
@@ -37,7 +38,9 @@
             chunk!
             chunk-for-each!
 
-            delete-duplicates/sort!))
+            delete-duplicates/sort!
+
+            get-gc-metrics-updater))
 
 (define (call-with-time-logging action thunk)
   (simple-format #t "debug: Starting ~A\n" action)
@@ -243,3 +246,30 @@
                           current-element
                           (cons current-element
                                 result)))))))))
+
+(define (get-gc-metrics-updater registry)
+  (define metrics
+    `((gc-time-taken
+       . ,(make-gauge-metric registry "guile_gc_time_taken"))
+      (heap-size
+       . ,(make-gauge-metric registry "guile_heap_size"))
+      (heap-free-size
+       . ,(make-gauge-metric registry "guile_heap_free_size"))
+      (heap-total-allocated
+       . ,(make-gauge-metric registry "guile_heap_total_allocated"))
+      (heap-allocated-since-gc
+       . ,(make-gauge-metric registry "guile_allocated_since_gc"))
+      (protected-objects
+       . ,(make-gauge-metric registry "guile_gc_protected_objects"))
+      (gc-times
+       . ,(make-gauge-metric registry "guile_gc_times"))))
+
+  (lambda ()
+    (let ((stats (gc-stats)))
+      (for-each
+       (match-lambda
+         ((name . metric)
+          (let ((value (assq-ref stats name)))
+            (metric-set metric value))))
+       metrics))))
+

guix-data-service/web/controller.scm

@@ -149,7 +149,11 @@
                                              "-"
                                              "_"))
                                            #:labels '(name))))
-                                       pg-stat-indexes-fields)))
+                                       pg-stat-indexes-fields))
+
+         (gc-metrics-updater
+          (get-gc-metrics-updater registry)))
+
     (lambda ()
       (letpar& ((metric-values
                  (with-thread-postgresql-connection
@@ -234,6 +238,8 @@
                         (state            . ,state)))))
                   load-new-guix-revision-job-metrics)
 
+        (gc-metrics-updater)
+
         (list (build-response
                #:code 200
                #:headers '((content-type . (text/plain))))