Create README.md

Author: b-viguier

README.md

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+# newrelic-guzzle-http-host
+A workaround to report Guzzle requests with correct Host Http header in NewRelic.
+
+In some case, you may use a common proxy for several external calls to various hosts,
+using the standard [`Host`](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Host) to route your requests.
+Unfortunatly, since all of these calls use the same uri, NewRelic will report them as external calls to the same target.
+
+This small script provide a workaround to report the correct endpoint according to the HTTP `Host` header.
+
+:warning: This library is a *workaround* relying on the current implementation of the newrelic extension!
+Test it carefully before to use it with a new extension release.
+See explanations section below for more details.
+
+ℹ️An official feature request has been submitted to NewRelic team.
+
+## Installation
+
+```
+composer require bedrockstreaming/newrelic-guzzle-http-host
+```
+That's all! The script is automatically imported thanks to [Composer autoloader](https://getcomposer.org/doc/04-schema.md#files).
+
+ℹ️ You can also copy the content of [`newrelic_hook.php`](newrelic_hook.php) directly in your project.
+
+## Explanations
+
+The [newrelic extension](https://github.com/newrelic/newrelic-php-agent) [defines a Php function](https://github.com/newrelic/newrelic-php-agent/blob/3f93ee47f80703d46d8fccd53be7d6b80361a594/agent/lib_guzzle6.c#L433-L461),
+a [Guzzle](https://github.com/guzzle/guzzle) middleware to report some metrics on each requests sent from Php.
+
+
+Fortunately, the extension defines this function only if it doesn't already exist… then this workaround define this function before newrelic does 😅.
+Original code is copied in this function, but we create a new http requests dedicated to NewRelic reporting, resolving the Http `HOST` header.
+The original request used by Guzzle stays unchanged.
+