docs: `Runtime` isn't rendered on docs.rs

see: https://docs.rs/kubert-prometheus-tokio/latest/kubert_prometheus_tokio/

information about how to configure docs.rs builds is available at: https://docs.rs/about/metadata

```
[package.metadata.docs.rs]
# Additional `RUSTFLAGS` to set (default: [])
rustc-args = ["--example-rustc-arg"]
```

we could pass `tokio_unstable` here so that the `Runtime` struct is rendered on docs.rs.