Add simple stats API

[junr03]

This PR exposes the concept of Metrics with a high level API. It takes liberal inspiration from the Simple<Counter,Gauge,Histogram> in the CPP SDK.

Users of the Rust SDK will now be able to create and manipulate simple Counters, Gauges, and Histograms that exist in the Envoy Runtime.

Note that this change is more generic than https://github.com/proxy-wasm/proxy-wasm-rust-sdk/pull/229 given that it exposes the concept of Counters, Gauges, and Histograms as a first class implementation, rather than interacting directly with the metrics host calls via the Root Context.

cc @casimiro