fix panic in prometheus when hostname field is missing

Signed-off-by: Thomas Labarussias <[email protected]>

README.md

@@ -1427,7 +1427,7 @@ The name from the table should match with the `hypertable` output configuration.
 Run you daemon and try (from Falco's documentation) :
 
 ```bash
-curl -XPOST "http://localhost:2801/" -d'{"output":"16:31:56.746609046: Error File below a known binary directory opened for writing (user=root command=touch /bin/hack file=/bin/hack)","priority":"Error","rule":"Write below binary dir","time":"2019-05-17T15:31:56.746609046Z", "output_fields": {"evt.time":1507591916746609046,"fd.name":"/bin/hack","proc.cmdline":"touch /bin/hack","user.name":"root"}}'
+curl -XPOST "http://localhost:2801/" -d'{"output":"16:31:56.746609046: Error File below a known binary directory opened for writing (user=root command=touch /bin/hack file=/bin/hack)","hostname": "localhost", "priority":"Error","rule":"Write below binary dir","time":"2019-05-17T15:31:56.746609046Z", "output_fields": {"evt.time":1507591916746609046,"fd.name":"/bin/hack","proc.cmdline":"touch /bin/hack","user.name":"root"}}'
 ```
 
 You should get :

handlers.go

@@ -136,6 +136,8 @@ func newFalcoPayload(payload io.Reader) (types.FalcoPayload, error) {
 	promLabels := map[string]string{"rule": falcopayload.Rule, "priority": falcopayload.Priority.String(), "k8s_ns_name": kn, "k8s_pod_name": kp}
 	if falcopayload.Hostname != "" {
 		promLabels["hostname"] = falcopayload.Hostname
+	} else {
+		promLabels["hostname"] = "unknown"
 	}
 
 	for key, value := range config.Customfields {