…eader configurable (mpdude)This PR was squashed before being merged into the 4.3-dev branch (closes#30964).Discussion----------[HttpKernel] Add a "short" trace header format, make header configurable| Q             | A| ------------- | ---| Branch?       | master| Bug fix?      | no| New feature?  | yes| BC breaks?    | no| Deprecations? | no| Tests pass?   | yes| Fixed tickets || License       | MIT| Doc PR        |symfony/symfony-docs#11340This pull requests adds the first usage of `array_key_first` to the Symfony code base. Additionally, it makes it possible to configure the `HttpCache` to also add a "trace" header in production.The `HttpCache` is a convenient, low-barrier yet performant way of accelerating the application. By having the "trace" information returned as a header in production as well, you can write it to server log files. For example, with Apache you can use `%{X-Symfony-Cache}o` in the `LogFormat` directive to log response headers.With the information in the log files, you can easily process it from logfile processing/system metrics tools to find out about cache performance, efficiency and the URLs that might need extra cache tweaking.<img width="1040" alt="Bildschirmfoto 2019-04-07 um 11 43 23" src="https://user-images.githubusercontent.com/1202333/55681763-6e90e980-592a-11e9-900f-e096350531c2.png">The "short" format will only output information for the main request to avoid leaking internal URLs for ESI subrequests. I also chose a concise format like `stale/valid/store` because that's much easier to parse out of logfiles (no whitespace, no need for quotes etc.).If you're not comfortable with having `Symfony` in the header name that way, the header name can be changed through a configuration setting as well.#FOSSHackathonCommits-------9a2fcc9 [HttpKernel] Add a \"short\" trace header format, make header configurable

… config options in `HttpCache` (mpdude)This PR was squashed before being merged into the master branch (closes#11340).Discussion----------[HttpKernel] Document `trace_level` and `trace_header` config options in `HttpCache`Here's a small addition forsymfony/symfony#30964.#FOSSHackathonCommits-------75febdc [HttpKernel] Document `trace_level` and `trace_header` config options in `HttpCache`