added monitoring of event loop time

should indicate if node is blocking on libuv threads
as described in https://nodejs.org/api/dns.html#dns_dns_lookup

libraries/metrics/event_loop.coffee

@@ -0,0 +1,18 @@
+seconds = 1000
+
+module.exports = EventLoopMonitor =
+	monitor: (logger) ->
+		interval = setInterval () ->
+			EventLoopMonitor.Delay()
+		, 1 * seconds
+		Metrics = require "./metrics"
+		Metrics.registerDestructor () ->
+			clearInterval(interval)
+
+	Delay: () ->
+		Metrics = require "./metrics"
+		t1 = process.hrtime()
+		setImmediate () ->
+			delta = process.hrtime(t1)
+			responseTime = delta[0]*1e6 + delta[1]*1e-3
+			Metrics.timing("event-loop-microsec", responseTime)

libraries/metrics/metrics.coffee

@@ -39,6 +39,7 @@ module.exports =
 	mongodb: require "./mongodb"
 	http: require "./http"
 	open_sockets: require "./open_sockets"
+	event_loop: require "./event_loop"
 
 	close: () ->
 		for func in destructors