Fix metricbeat filesystem metric collection

Add a root filesystem mount point for metricbeat to
enable access to the host file systems for collecting
filesystem specific metrics.

The filesystems are mounted under /hostfs within the
beat Pod, therefore the system.filesystem.mount_point
value must be filtered by Logstash to rewrite the path
to the original mount path on the host.

Change-Id: I7d98560eb00928e39a1c97ebc3e6b5dda8f5b278
Story: 2005733
Task: 37291
Signed-off-by: Matt Peters <matt.peters@windriver.com>

stx-monitor-helm/stx-monitor-helm/manifests/monitor_manifest.yaml

@@ -507,6 +507,14 @@ data:
             - type: kubernetes
               host: ${NODE_NAME}
               hints.enabled: true
+    extraVolumeMounts:
+      - name: root
+        mountPath: /hostfs
+        readOnly: true
+    extraVolumes:
+      - name: root
+        hostPath:
+          path: /
   source:
     type: tar
     location: http://172.17.0.1:8080/helm_charts/starlingx/metricbeat-1.6.0.tgz
@@ -701,6 +709,16 @@ data:
             type => 'beats'
           }
         }
+    filters:
+      main: |-
+        filter {
+          mutate {
+            gsub => [
+              # strip /hostfs prefix from mount points in metricbeat metrics
+              "[system][filesystem][mount_point]", "^/hostfs($|/)", "/"
+            ]
+          }
+        }
     ports:
       # influxdb port conflict, can't use 25826
       # - containerPort: 25826