Fix problem with metrics when ping return Destination net unreachable ( windows ) (#1561)

* Fix problem with metrics when ping return Destination net unreachable
Add test case TestUnreachablePingGather
Add percent_reply_loss
Fix some other tests

* Add errors measurment

* fir problem with ping reply "TTL expired in transit" ( use regex for more specific condition - TTL in line but it's a not valid replay )
add test case for "TTL expired in transit" - TestTTLExpiredPingGather

plugins/inputs/ping/README.md

@@ -0,0 +1,36 @@
+# Ping input plugin
+
+This input plugin will measures the round-trip
+
+## Windows:
+### Configration:
+```
+	## urls to ping
+	urls = ["www.google.com"] # required
+	
+	## number of pings to send per collection (ping -n <COUNT>)
+	count = 4 # required
+	
+	## Ping timeout, in seconds. 0 means default timeout (ping -w <TIMEOUT>)
+	Timeout = 0
+```
+### Measurements & Fields:
+- packets_transmitted ( from ping output )
+- reply_received ( increasing only on valid metric from echo replay, eg. 'Destination net unreachable' reply will increment packets_received but not reply_received )
+- packets_received ( from ping output )
+- percent_reply_loss ( compute from packets_transmitted and reply_received )
+- percent_packets_loss ( compute from packets_transmitted and packets_received )
+- errors ( when host can not be found or wrong prameters is passed to application )
+- response time
+    - average_response_ms ( compute from minimum_response_ms and maximum_response_ms )
+    - minimum_response_ms ( from ping output )
+    - maximum_response_ms ( from ping output )
+	
+### Tags:
+- server
+
+### Example Output:
+```
+* Plugin: ping, Collection 1
+ping,host=WIN-PBAPLP511R7,url=www.google.com average_response_ms=7i,maximum_response_ms=9i,minimum_response_ms=7i,packets_received=4i,packets_transmitted=4i,percent_packet_loss=0,percent_reply_loss=0,reply_received=4i 1469879119000000000
+```