counters: add counters for DNS resolution.

Author: Christian Hergert

src/mongoc/mongoc-client.c

@@ -118,6 +118,7 @@ mongoc_client_connect_tcp (const mongoc_uri_t       *uri,
    s = getaddrinfo (host->host, portstr, &hints, &result);
 
    if (s != 0) {
+      mongoc_counter_dns_failure_inc ();
       bson_set_error(error,
                      MONGOC_ERROR_STREAM,
                      MONGOC_ERROR_STREAM_NAME_RESOLUTION,
@@ -126,6 +127,8 @@ mongoc_client_connect_tcp (const mongoc_uri_t       *uri,
       RETURN (NULL);
    }
 
+   mongoc_counter_dns_success_inc ();
+
    for (rp = result; rp; rp = rp->ai_next) {
       /*
        * Create a new non-blocking socket.

src/mongoc/mongoc-counters.defs

@@ -59,3 +59,7 @@ COUNTER(protocol_ingress_error, "Protocol",     "Ingress Errors",      "The numb
 
 COUNTER(auth_failure,           "Auth",         "Failures",            "The number of failed authentication requests.")
 COUNTER(auth_success,           "Auth",         "Success",             "The number of successful authentication requests.")
+
+
+COUNTER(dns_failure,            "DNS",          "Failure",             "The number of failed DNS requests.")
+COUNTER(dns_success,            "DNS",          "Success",             "The number of successful DNS requests.")