[PATCH] Ensure agent is always set for mysql/grpc (#343)

If either of these modules switch to lazily importing any of our patched
files, we would fail to set the agent in some cases.

PR-URL: googleapis/cloud-trace-nodejs#343

src/hooks/userspace/hook-grpc.js

@@ -431,7 +431,9 @@ module.exports = function(version_, agent_) {
   return {
     'src/node/src/client.js': {
       patch: function(client) {
-        agent = agent_;
+        if (!agent) {
+          agent = agent_;
+        }
         shimmer.wrap(client, 'makeClientConstructor',
             makeClientConstructorWrap);
       },
@@ -445,6 +447,9 @@ module.exports = function(version_, agent_) {
     },
     'src/node/src/server.js': {
       patch: function(server) {
+        if (!agent) {
+          agent = agent_;
+        }
         shimmer.wrap(server.Server.prototype, 'register', serverRegisterWrap);
       },
       unpatch: function(server) {

src/hooks/userspace/hook-mysql.js

@@ -86,7 +86,9 @@ module.exports = function(version_, agent_) {
   return {
     'lib/Connection.js': {
       patch: function(Connection) {
-        agent = agent_;
+        if (!agent) {
+          agent = agent_;
+        }
         shimmer.wrap(Connection, 'createQuery', createQueryWrap);
       },
       unpatch: function(Connection) {
@@ -96,6 +98,9 @@ module.exports = function(version_, agent_) {
     },
     'lib/Pool.js': {
       patch: function(Pool) {
+        if (!agent) {
+          agent = agent_;
+        }
         shimmer.wrap(Pool.prototype, 'getConnection', wrapGetConnection);
       },
       unpatch: function(Pool) {