Remove locking in call-with-duration-metric

As I think this is unnecessary.

prometheus.scm

@@ -424,20 +424,13 @@ The metric with the name @var{metric-name} is fetched from the
 "
   (let* ((metric
           (or (metrics-registry-fetch-metric registry metric-name)
-              (call-with-blocked-asyncs
-               (lambda ()
-                 (monitor
-                  ;; Check once more in case another thread has created
-                  ;; the metric while this thread was waiting for the
-                  ;; mutex
-                  (or (metrics-registry-fetch-metric registry metric-name)
-                      (make-histogram-metric
-                       registry
-                       metric-name
-                       #:buckets buckets
-                       #:docstring docstring
-                       #:labels labels
-                       #:label-preset-values label-preset-values)))))))
+              (make-histogram-metric
+               registry
+               metric-name
+               #:buckets buckets
+               #:docstring docstring
+               #:labels labels
+               #:label-preset-values label-preset-values)))
          (start-time (get-internal-real-time)))
     (call-with-values
         thunk