Don't set UV_THREADPOOL_SIZE if already set

libraries/metrics/uv_threadpool_size.coffee

@@ -1,2 +1,3 @@
-process.env.UV_THREADPOOL_SIZE=16
-console.log "Set UV_THREADPOOL_SIZE=#{process.env.UV_THREADPOOL_SIZE}"
+unless process.env.UV_THREADPOOL_SIZE
+	process.env.UV_THREADPOOL_SIZE=16
+	console.log "Set UV_THREADPOOL_SIZE=#{process.env.UV_THREADPOOL_SIZE}"