refactor(collector): have Registry:encode do the encoding

[mxinden]

Let's face it, the Collector trait I designed in #82 is too complex.

Instead of having Registry provide access to its inner Metrics, have
Registry encode the Metrics directly via Registry::encode. In the same
vain, have Collector::encode encode the collected Metrics directly.

This allows for a much simpler Collector trait, removing all complexity due to
providing access to a Metric externally.

/// The [`Collector`] abstraction allows users to provide additional metrics and
/// their description on each scrape.
///
/// An example use-case is an exporter that retrieves a set of operating system metrics
/// ad-hoc on each scrape.
///
/// Register a [`Collector`] with a [`Registry`](crate::registry::Registry) via
/// [`Registry::register_collector`](crate::registry::Registry::register_collector).
///
/// ```
/// # use prometheus_client::metrics::counter::ConstCounter;
/// # use prometheus_client::collector::Collector;
/// # use prometheus_client::encoding::{DescriptorEncoder, EncodeMetric};
/// #
/// #[derive(Debug)]
/// struct MyCollector {}
///
/// impl Collector for MyCollector {
///     fn encode(&self, mut encoder: DescriptorEncoder) -> Result<(), std::fmt::Error> {
///         let counter = ConstCounter::new(42);
///         let metric_encoder = encoder.encode_descriptor(
///             "my_counter",
///             "some help",
///             None,
///             counter.metric_type(),
///         )?;
///         counter.encode(metric_encoder)?;
///         Ok(())
///     }
/// }
/// ```
pub trait Collector: std::fmt::Debug + Send + Sync + 'static {
    /// Once the [`Collector`] is registered, this method is called on each scrape.
    fn encode<'a>(&'a self, encoder: DescriptorEncoder) -> Result<(), std::fmt::Error>;
}

[mxinden]

@thomaseizinger based on your comments in #82 (comment) I think you will like this change.

[thomaseizinger]

I think I was able to simplify this a bit in mxinden#2.

[gurinderu]

@mxinden looks like there is no way to add custom labels with the new API. Probably we should add an additional encode_descriptor function with optional labels. WDYT?

[mxinden]

Does the encode_family method solve your use-case? Example:

https://github.com/libp2p/rust-libp2p/blob/cddc306978a7cca705de5ce10ec5e16e8b732084/misc/metrics/src/identify.rs#L211-L223