health-metric: add dbus path for read-write storage

Add the DBus path for read write storage to be used in phosphor-health-monitor.

Change-Id: Ic162468dc272a6707cd39476d8e9f128de83e874
Signed-off-by: Jagpal Singh Gill <paligill@gmail.com>
diff --git a/yaml/xyz/openbmc_project/Metric/Value.interface.yaml b/yaml/xyz/openbmc_project/Metric/Value.interface.yaml
index 0743bae..7fb491a 100644
--- a/yaml/xyz/openbmc_project/Metric/Value.interface.yaml
+++ b/yaml/xyz/openbmc_project/Metric/Value.interface.yaml
@@ -89,6 +89,10 @@
             description: >
                 The user CPU utilization.
             value: cpu/user
+          - name: ReadWriteStorage
+            description: >
+                The available read write storage.
+            value: storage/rw
           - name: RebootCount
             description: >
                 Reboot statistic to track number of times device rebooted