[metrics-rs-integration / task] Add metrics.rs integration for `TaskMetrics`

[jlizen]

#68 added an integration with metrics.rs for RuntimeMetrics.

We should add similar for TaskMetrics. it will require very similar code. Probably we should refactor slightly to generalize some of the macros, but up to the author.

Just as users that want an easy way to export RuntimeMetrics need the integration, so to do TaskMetrics users.

A specific use case that affects an application I work with: We want to instrument our Hyper server's accept loop separate from general runtime metrics.

There is a fair bit of boilerplate needed today to export those metrics. Simplifying that story with a metrics.rs integration will make them much more accessible.