This change culminates recent work to restructure the balancer to use a
PoolQueue so that balancer changes may occur independently of request
processing. This replaces independent discovery buffering so that the
balancer task is responsible for polling discovery streams without
independent buffering. Requests are buffered and processed as soon as
the pool has available backends. Fail-fast circuit breaking is enforced
on the balancer's queue so that requests can't get stuck in a queue
indefinitely.

In general, the new balancer is instrumented directly with metrics, and
the relevant metric name prefix and labelset is provided by the stack.
In addition to detailed queue metrics including request (in-queue)
latency histograms, but also failfast states, discovery updates counts,
and balancer endpoint pool sizes.

---

* outbound: Move queues into the concrete stack (linkerd/linkerd2-proxy#2539)
* metrics: Remove unused features (linkerd/linkerd2-proxy#2542)
* Add the PoolQueue middleware (linkerd/linkerd2-proxy#2540)
* ci: Fixup codecov config (linkerd/linkerd2-proxy#2545)
* ci: Cancel prior runs (linkerd/linkerd2-proxy#2546)
* ci: Skip ARM builds during non-release CI (linkerd/linkerd2-proxy#2547)
* deps: Update tokio, tonic, and prost (linkerd/linkerd2-proxy#2544)
* build(deps): bump tj-actions/changed-files from 40.2.0 to 40.2.1 (linkerd/linkerd2-proxy#2549)
* metrics: Use prometheus-client for proxy_build_info (linkerd/linkerd2-proxy#2551)
* balance: Add a p2c Pool implementation (linkerd/linkerd2-proxy#2541)
* metrics: Export process metrics using prometheus-client (linkerd/linkerd2-proxy#2552)
* linkerd_identity: split `linkerd_identity::Id` into DNS and URI variants (linkerd/linkerd2-proxy#2538)
* outbound: Move HTTP balancer into its own module (linkerd/linkerd2-proxy#2554)
* app: Setup prom registry for use in balancers (linkerd/linkerd2-proxy#2555)
* vscode: Move workspace settings to devcontainer (linkerd/linkerd2-proxy#2557)
* build(deps): bump tj-actions/changed-files from 40.2.1 to 40.2.2 (linkerd/linkerd2-proxy#2556)
* balance: Instrument metrics in pool balancer (linkerd/linkerd2-proxy#2558)
* Enable PoolQueue balancer (linkerd/linkerd2-proxy#2559)

Signed-off-by: Oliver Gould <ver@buoyant.io>

This change culminates recent work to restructure the balancer to use a
PoolQueue so that balancer changes may occur independently of request
processing. This replaces independent discovery buffering so that the
balancer task is responsible for polling discovery streams without
independent buffering. Requests are buffered and processed as soon as
the pool has available backends. Fail-fast circuit breaking is enforced
on the balancer's queue so that requests can't get stuck in a queue
indefinitely.

In general, the new balancer is instrumented directly with metrics, and
the relevant metric name prefix and labelset is provided by the stack.
In addition to detailed queue metrics including request (in-queue)
latency histograms, but also failfast states, discovery updates counts,
and balancer endpoint pool sizes.

---

* outbound: Move queues into the concrete stack (linkerd/linkerd2-proxy#2539)
* metrics: Remove unused features (linkerd/linkerd2-proxy#2542)
* Add the PoolQueue middleware (linkerd/linkerd2-proxy#2540)
* ci: Fixup codecov config (linkerd/linkerd2-proxy#2545)
* ci: Cancel prior runs (linkerd/linkerd2-proxy#2546)
* ci: Skip ARM builds during non-release CI (linkerd/linkerd2-proxy#2547)
* deps: Update tokio, tonic, and prost (linkerd/linkerd2-proxy#2544)
* build(deps): bump tj-actions/changed-files from 40.2.0 to 40.2.1 (linkerd/linkerd2-proxy#2549)
* metrics: Use prometheus-client for proxy_build_info (linkerd/linkerd2-proxy#2551)
* balance: Add a p2c Pool implementation (linkerd/linkerd2-proxy#2541)
* metrics: Export process metrics using prometheus-client (linkerd/linkerd2-proxy#2552)
* linkerd_identity: split `linkerd_identity::Id` into DNS and URI variants (linkerd/linkerd2-proxy#2538)
* outbound: Move HTTP balancer into its own module (linkerd/linkerd2-proxy#2554)
* app: Setup prom registry for use in balancers (linkerd/linkerd2-proxy#2555)
* vscode: Move workspace settings to devcontainer (linkerd/linkerd2-proxy#2557)
* build(deps): bump tj-actions/changed-files from 40.2.1 to 40.2.2 (linkerd/linkerd2-proxy#2556)
* balance: Instrument metrics in pool balancer (linkerd/linkerd2-proxy#2558)
* Enable PoolQueue balancer (linkerd/linkerd2-proxy#2559)

Signed-off-by: Oliver Gould <ver@buoyant.io>