get rid of some bashisms / clean up. add ping metric. add --metrics cli option

metrics/heartbeat.sh

@@ -0,0 +1,9 @@
+#!/bin/sh
+
+collect () {
+  report 1
+}
+
+docs () {
+  echo "Send a simple heartbeat in form of an integer '1'."
+}

metrics/network_io.sh

@@ -8,7 +8,7 @@ init () {
       NETWORK_IO_INTERFACE="eth0"
     fi
   fi
-  readonly __network_io_divisor=$[$INTERVAL * 1024]
+  readonly __network_io_divisor=$(($INTERVAL * 1024))
 }
 
 if is_osx; then

metrics/ping.sh

@@ -1,9 +1,23 @@
 #!/bin/sh
 
+init () {
+  if [ -z $PING_REMOTE_HOST ]; then
+    echo "Error: ping metric requires \$PING_REMOTE_HOST to be specified"
+    exit 1
+  fi
+}
+
 collect () {
-  report 1
+  ping -c 1 $PING_REMOTE_HOST > /dev/null 2>&1
+  if [ $? -eq 0 ]; then
+    report 1
+  else
+    report 0
+  fi
 }
 
 docs () {
-  echo "Send a simple ping in form of an integer '1'."
+  echo "Check if remote host is reachable by sending a single ping."
+  echo "Reports '1' if ping was successful, '0' if not."
+  echo "\$PING_REMOTE_HOST="
 }