Counter API is overly restrictive

Currently Counter's API is:
https://github.com/prometheus/client_rust/blob/master/src/metrics/counter.rs#L76-L83

    /// Increase the [`Counter`] by 1, returning the previous value.
    pub fn inc(&self) -> N {
        self.value.inc()
    }

    /// Increase the [`Counter`] by `v`, returning the previous value.
    pub fn inc_by(&self, v: N) -> N {
        self.value.inc_by(v)
    }

which requires inc and inc_by to return the previous value. This is rarely needed for metric collecting use-case, and it hinders the ability to use per-thread thread-local accumulator to prevent contention (like this) and increase performance.

Start by reviewing src/metrics/counter.rs at lines 76-83 and the linked hytra::TrAdder design. Determine the intended Counter increment API and how its return-value requirement affects thread-local accumulation and contention. Done should include an agreed API shape and coverage for the changed increment behavior.