Docs: kube prometheus jsonnet (#198)

Signed-off-by: Christian Beneke <christian.beneke@traderepublic.com>

troubleshooting/custom-prometheus.md

@@ -4,6 +4,8 @@ When using custom prometheus with Lens app, Lens expects certain things for prom
 
 ## kube-prometheus
 
+### Manual
+
 1. To see node metrics properly, please add
 
 ```
@@ -27,6 +29,39 @@ metricRelabelings:
   targetLabel: instance
 ```
 
+### Jsonnet
+
+The required label replacements are bundled in [jsonnet/custom-prometheus](../jsonnet/custom-prometheus.jsonnet). To install it copy the file or use
+[Jsonnet Bundler](https://github.com/jsonnet-bundler/jsonnet-bundler). For jsonnet bundler add the following dependency to your `jsonnetfile.json`:
+
+```
+{
+  "name": "lens",
+  "source": {
+    "git": {
+      "remote": "https://github.com/lensapp/lens",
+      "subdir": "jsonnet"
+    }
+  },
+  "version": "master"
+}
+```
+
+and run `jb install`. When the installation was successful include it into your definitions. Using the [example](https://github.com/coreos/kube-prometheus#compiling)
+of kube-prometheus, e.g.:
+
+```
+local kp =
+  (import 'kube-prometheus/kube-prometheus.libsonnet') +
+  (import 'lens/custom-prometheus.jsonnet') +
+  {
+    _config+:: {
+      namespace: 'monitoring',
+    },
+  };
+...
+```
+
 ## Helm chart
 
 1. To see cpu metrics properly, please set value of `server.global.scrape_timeout` less than 1 minute, for example