Update dashboard.json to use dynamic rate interval

Per https://grafana.com/blog/2020/09/28/new-in-grafana-7.2-__rate_interval-for-prometheus-rate-queries-that-just-work/, a fixed rate interval works only in some cases. The current `1m` assumes that everyone uses the default prometheus scraping interval (15s), but this is not guaranteed, so replace with `$__rate_interval` which should deal with things automatically in most cases.

dashboard.json

@@ -173,7 +173,7 @@
             "uid": "${DS_PROMETHEUS}"
           },
           "editorMode": "code",
-          "expr": "sum(rate(smokeping_response_duration_seconds_bucket{host=\"${target:raw}\"}[1m])) by (le)",
+          "expr": "sum(rate(smokeping_response_duration_seconds_bucket{host=\"${target:raw}\"}[$__rate_interval])) by (le)",
           "format": "heatmap",
           "intervalFactor": 1,
           "legendFormat": "{{le}}",
@@ -549,4 +549,4 @@
   "uid": "i5aRaLaik",
   "version": 12,
   "weekStart": ""
-}
+}