proxy: v2.86.0 (#4075)

This release includes the results from continued profiling & performance
analysis. In addition to modifying internals to prevent unwarranted
memory growth, we've introduced new metrics to aid in debugging and
diagnostics: a new `request_errors_total` metric exposes the number of
requests that receive synthesized responses due to proxy errors; and a
suite of `stack_*` metrics expose proxy internals that can help us
identify unexpected behavior.

---

* trace: update `tracing-subscriber` dependency to 0.2.1 (linkerd/linkerd2-proxy#426)
* Reimplement the Lock middleware with tokio::sync (linkerd/linkerd2-proxy#427)
* Add the request_errors_total metric (linkerd/linkerd2-proxy#417)
* Expose the number of service instances in the proxy (linkerd/linkerd2-proxy#428)
* concurrency-limit: Share a limit across Services (linkerd/linkerd2-proxy#429)
* profiling: add benchmark and profiling scripts (linkerd/linkerd2-proxy#406)
* http-box: Box HTTP payloads via middleware (linkerd/linkerd2-proxy#430)
* lock: Generalize to protect a guarded value (linkerd/linkerd2-proxy#431)

.proxy-version

@@ -1 +1 @@
-v2.85.0
+v2.86.0