The tower-balance crate includes the `Load` and `Instrument` traits,
which are likely useful outside of balancers; and certainly have no
tight coupling with any specific balancer implementation. This change
extracts these protocol-agnostic traits into a dedicated crate.
The `Load` trait includes a latency-aware _PeakEWMA_ load strategy as
well as a simple _PendingRequests_ strategy for latency-agnostic
applications.
The `Instrument` trait is used by both of these strategies to track
in-flight requests without knowing protocol details. It is expected that
protocol-specific crates will provide, for instance, HTTP
time-to-first-byte latency strategies.
A default `NoInstrument` implementation tracks the a request until its
response future is satisfied.
This crate should only be published once tower-balance is published.
Part of https://github.com/tower-rs/tower/issues/286
Oliver Gould