0.4.2 release

[jlizen]

We have a couple doc fixes pending that will require a new crates.io release to be reflected in docs.rs:

• fix: rt: in docs.rs, annotate the module as requiring metrics-rs-integration along with rt and tokio_unstable #78
• docs: fix links #79

Giving it ~ a week to bake and pick up any feature changes, else we can cut the release.