Example with option to modify alert thresholds

Review comment: https://github.com/coreos/kube-prometheus/pull/425#discussion_r387494885

examples/weavenet-example.jsonnet

@@ -1,9 +1,39 @@
 local kp =  (import 'kube-prometheus/kube-prometheus.libsonnet') +
-            (import 'kube-prometheus/kube-prometheus-weavenet.libsonnet');
+            (import 'kube-prometheus/kube-prometheus-weavenet.libsonnet') + {
+  _config+:: {
+    namespace: 'monitoring',
+  },
+  prometheusAlerts+:: {
+    groups: std.map(
+      function(group)
+        if group.name == 'weave-net' then
+          group {
+            rules: std.map(function(rule)
+              if rule.alert == "WeaveNetFastDPFlowsLow" then
+                rule {
+                  expr: "sum(weave_flows) < 2000"
+                }
+              else if rule.alert == "WeaveNetIPAMUnreachable" then
+                rule {
+                  expr: "weave_ipam_unreachable_percentage > 25"
+                }
+              else
+                rule
+              ,
+              group.rules
+            )
+          }
+        else
+          group,
+        super.groups
+      ),
+  },
+};
 
 { ['00namespace-' + name]: kp.kubePrometheus[name] for name in std.objectFields(kp.kubePrometheus) } +
 { ['0prometheus-operator-' + name]: kp.prometheusOperator[name] for name in std.objectFields(kp.prometheusOperator) } +
 { ['node-exporter-' + name]: kp.nodeExporter[name] for name in std.objectFields(kp.nodeExporter) } +
 { ['kube-state-metrics-' + name]: kp.kubeStateMetrics[name] for name in std.objectFields(kp.kubeStateMetrics) } +
 { ['prometheus-' + name]: kp.prometheus[name] for name in std.objectFields(kp.prometheus) } +
-{ ['prometheus-adapter-' + name]: kp.prometheusAdapter[name] for name in std.objectFields(kp.prometheusAdapter) }
+{ ['prometheus-adapter-' + name]: kp.prometheusAdapter[name] for name in std.objectFields(kp.prometheusAdapter) } +
+{ ['grafana-' + name]: kp.grafana[name] for name in std.objectFields(kp.grafana) }