Prefix logging for BSP

[alexarchambault]

When Mill runs as a BSP server, this makes it log in a similar way as when running Mill from the command-line, with […] prefixes.

For example, the small BSP session run in the test added in this PR logs the following to stderr:

[bsp] Trying to load BSP server...
[bsp] BSP server started
[1-buildInitialize] Entered buildInitialize
[1-buildInitialize] buildInitialize took 2 msec
[2-workspaceBuildTargets] Entered workspaceBuildTargets
[bsp-init-build.mill-61] [info] compiling 3 Scala sources to /Users/alexandre/projects/mill/mill-class-loading/out/integration/ide/bsp-server/local/daemon/testForked.dest/sandbox/run-1/out/mill-build/compile.dest/classes ...
[bsp-init-build.mill-61] [warn] package scala contains object and package with same name: caps.
[bsp-init-build.mill-61] [warn] This indicates that there are several versions of the Scala standard library on the classpath.
[bsp-init-build.mill-61] [warn] The build should be reconfigured so that only one version of the standard library is on the classpath.
[bsp-init-build.mill-61] [warn] one warning found
[bsp-init-build.mill-61] [info] done compiling
[bsp-init] SNAPSHOT
[2-workspaceBuildTargets] Evaluating 5 tasks
[2-workspaceBuildTargets] Done
[2-workspaceBuildTargets] Evaluating 1 task
[2-workspaceBuildTargets] Done
[2-workspaceBuildTargets] workspaceBuildTargets took 4197 msec
[3-loggingTest] Entered loggingTest
[3-loggingTest] Evaluating 1 task
[3-loggingTest-1] bspLoggingTest from System.out
[3-loggingTest-1] bspLoggingTest from System.err
[3-loggingTest-1] bspLoggingTest from Console.out
[3-loggingTest-1] bspLoggingTest from Console.err
[3-loggingTest-1] bspLoggingTest from Task.log.info
[3-loggingTest] Done
[3-loggingTest] loggingTest took 11 msec
[4-buildShutdown] Entered buildShutdown
[5-onBuildExit] Entered onBuildExit
Reload finished, result: mill.api.internal.BspServerResult$Shutdown$@6726cc69
Shutting down executor
[bsp] BSP session returned with mill.api.internal.BspServerResult$Shutdown$@6726cc69
Stopping server via handle...
[bsp] Exiting BSP runner loop. Stopping BSP server. Last result: Success(mill.api.internal.BspServerResult$Shutdown$@6726cc69)

This helps make sense of what's going on when working on BSP-releated features, but could also help Mill BSP users in case anything goes wrong.

[alexarchambault]

Includes #5153 for now

[alexarchambault]

About 1113602, without it, it seems System.{out,err} don't get captured in tasks run by the BSP server. In the example above, bspLoggingTest from System.out and bspLoggingTest from System.err get no prefix in that case. I wasn't able to reproduce that issue outside of BSP. That's what motivated #5155 initially.

[lihaoyi]

Do we need these changes to the Logger interface?

Logger is shared throughout Mill as part of core.api, and thus it really needs to be as simple as possible. Rather than adding a more verbose version of info/debug/etc., could we instead instantiate sub-classes of Logger inside MillBuildServer that have the correct logKey set?

[lihaoyi]

Could we even use the PrefixLogger class for this? Adding a prefix to log lines is exactly what that class is meant to do

[lihaoyi]

Do we need custom names for these? We already take the sourcecode.Name of each endpoint method, which I would have thought would give us enough information about what endpoint each log message is comming from

[lihaoyi]

Let's remove this from this PR since it's unrelated

[lihaoyi]

Do we really need to call createLogger() and pass it in to every handlerRaw? Could we instead make handlerRaw take the sourcecode.Enclosing and create the logger internally?