Capture and log HTTP headers and payload

[outfoxer]

Is your feature request related to a problem? Please describe. HTTP headers and payload is a must for understanding issues causes

Describe the solution you'd like Capture and log HTTP headers and payload under http section of the transaction metadata

[Lansoweb]

We have some internal APIs and use the User-Agent header to identify the source of the request. On our Nodejs APIs the headers are present on Observability, but not for PHP and it's important to identify the source of a request while investigating an issue. We can use the setLabel feature, but it would be better to have native support for headers.

ElasticApm::getCurrentTransaction()->context()->setLabel('user-agent', $userAgent);