v2.86.0

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.