HttpStreamPool: Add experimental trace events

This CL add experimental trace events to HttpStreamPool subclasses.
Currently we use tracks for AttemptManager, TcpBasedAttempt and QuicTask
and these are connected with flows. We may change how to record these
trace events if we figure out better way to record events.

Introduce a new trace category since it might be noisy if we use the
existing "net" category.

Bug: 383824591
Change-Id: I85ef2b6ab2a2ab92e1ce9b654fdc59848057a62a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6437996
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: Stephen Nusko <nuskos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1446399}