metrics: Add graph legends.

* src/cuirass/templates.scm (global-metrics-content): Add legends.

src/cuirass/templates.scm

@@ -945,15 +945,25 @@ window.~a = new Chart\
         ,(apply map avg-eval-duration-row avg-eval-durations)))
       (br)
       (h6 "Build speed.")
+      (p "This shows the difference between newly added derivations and built
+derivations per day.")
       (canvas (@ (id ,builds-chart)))
       (br)
       (h6 "Evaluation average build start time.")
+      (p "This is the average time required for an evaluation to start its
+builds.")
+      (br)
       (canvas (@ (id ,build-start-chart)))
       (br)
       (h6 "Evaluation completion speed.")
+      (p "The evaluation completion speed is the sum of an evaluation
+completed builds divided by the time required to build them.")
+      (br)
       (canvas (@ (id ,evaluation-speed-chart)))
       (br)
       (h6 "Pending builds.")
+      (p "This is the sum of all the currently pending builds.")
+      (br)
       (canvas (@ (id ,pending-builds-chart)))
       (br)
       (h6 "Percentage of failed evaluations.")