least request lb: include pending requests

[benjawo]

Commit Message:

LeastRequestLoadBalancer ranks hosts by host.stats().rq_active_, which only counts streams attached in attachStreamToClient. Streams queued in pending_streams_ on a connection pool waiting for connection establishment are invisible, so a host with slow handshakes can keep winning picks until queued streams attach. See #44989.

Add a per-host rq_pending_active gauge tracked in PendingStream's ctor/dtor and have LeastRequestLoadBalancer fold it into the value it compares behind a runtime guard, default off
(envoy.reloadable_features.least_request_lb_count_pending_requests).

Fixes #44989

Additional Description: This was an AI assisted code change. I understand all code written.
Risk Level: Low
Testing: UTs. Local repro script against envoy process.
Docs Changes: N/A
Release Notes: New changelog file.
Platform Specific Features: N/A
[Optional Runtime guard:] envoy.reloadable_features.least_request_lb_count_pending_requests
[Optional Fixes #Issue] #44989
[Optional Fixes commit #PR or SHA] N/A
[Optional Deprecated:] N/A
[Optional API Considerations:] N/A

[repokitteh-read-only]

Hi @benjawo, welcome and thank you for your contribution.

We will try to review your Pull Request as quickly as possible.

In the meantime, please take a look at the contribution guidelines if you have not done so already.

🐱

Caused by: #45168 was opened by benjawo.

see: more, trace.

[repokitteh-read-only]

As a reminder, PRs marked as draft will not be automatically assigned reviewers,
or be handled by maintainer-oncall triage.

Please mark your PR as ready when you want it to be reviewed!

🐱

Caused by: #45168 was opened by benjawo.

see: more, trace.

[repokitteh-read-only]

CC @envoyproxy/runtime-guard-changes: FYI only for changes made to (source/common/runtime/runtime_features.cc).

🐱

Caused by: #45168 was ready_for_review by benjawo.

see: more, trace.