Colourize & pretty-print mock APM server's output

[MattAlp]

Prints output of all telemetry signals (logs, metrics, traces) nicely with colour in the terminal. Beats having to pipe everything into jq solely for readability.

[elasticsearchmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)

[DaveCTurner]

Not all outputs will support ANSI escape sequences, but I don't see anything which checks for that. In particular, this will mean you cannot simply take the raw output and process it with jq or jless any more. I expect it'll make it much harder to read logs captured in build scans too. Could you check we're logging to a terminal before enabling this?

[MattAlp]

I don't believe this is used for anything other than local sanity-testing / debugging telemetry emitted by ES during the dev cycle. It also explicitly does not get "logged", using Gradle's logging directly to print alongside the logging captured via ./gradlew run.

Previously, you'd have to grep ^{ | jq or something along those lines in order to catch the events and make them readable. However, making this opt-in and/or checking for terminal compatibility is a good idea.

[rjernst]

@MattAlp I think the point is the output might not being going to a terminal, it could be redirected to a file, as in ./gradlew run --witih-apm-server &> myrunlog.txt and searched later.

Unfortunately the JDK made this a little more difficult to detect starting in JDK 22. It used to be that System.console() == null was enough to detect a terminal, but with JDK 22+ System.console() always returns non-null. There is a method, Console.isTerminal(), but that's only available in JDK 22+. You can see logic for reflective logic to figure this out in Terminal.ConsoleTerminal.detectTerminal().

[MattAlp]

After speaking with David, it actually sounds like we don't want to rely on the mock server anyways & would prefer to have test-driven checks for tracing. I may re-open this PR later depending on the timeline to getting that functionality in.