Add Stats to XML-RPC Errors

[mdawaffe]

Changes proposed in this Pull Request:

• Track unique XML-RPC authentication error codes in a new Jetpack Option.
• Send those error codes as part of the heartbeat.

Sending a stat for each failed XML-RPC request might not scale for sites with too many errors. Sending a stat for each failed XML-RPC request might not scale for the stats system :)

Using the heartbeat makes it impossible to gauge how many XML-RPC errors occur, but it does allow us to determine how many sites experience XML-RPC errors, which is probably more useful for us.

Is this a new feature or does it add/remove features to an existing part of Jetpack?

No: improved telemetry.

Testing instructions:

1. Break your XML-RPC authentication :) For example, with the patch below.
2. Do something that will generate XML-RPC requests to your site. (API Dev Console, Calypso, …)
3. wp jetpack options get xmlrpc_errors - see the error code in the option
4. wp jetpack options update last_heartbeat 0 - to trick Jetpack into running the heartbeat on command.
5. wp cron event run jetpack_v2_heartbeat - to run the heartbeat on command.
6. wp jetpack options get xmlrpc_errors - see the option has been removed.
7. Wait a minute, then check that the stat has bumped. (If multiple people happen to be testing this around the same time, you may have to wait up to 5 minutes to see the stat bump results.)

diff --git a/packages/connection/src/Manager.php b/packages/connection/src/Manager.php
index b3d9b4ea9..dd7ea6eab 100644
--- a/packages/connection/src/Manager.php
+++ b/packages/connection/src/Manager.php
@@ -350,6 +350,7 @@ class Manager implements Manager_Interface {
 			$user_id = (int) $user_id;
 
 			$user = new \WP_User( $user_id );
+$user = false;
 			if ( ! $user || ! $user->exists() ) {
 				return new \WP_Error(
 					'unknown_user',

Proposed changelog entry for your changes:

• Improved error reporting for sites having connection trouble.

[jetpackbot]

	Warnings
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This is an automated check which relies on PULL_REQUEST_TEMPLATE. We encourage you to follow that template as it helps Jetpack maintainers do their job. If you think 'Testing instructions' or 'Proposed changelog entry' are not needed for your PR - please explain why you think so. Thanks for cooperation 🤖

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[jeherve]

This works well for me. 👍 Merging.

[dereksmart]

Beautiful, thank you!