feat: derive register logic on struct of metrics

One oftentimes has a struct of metrics like:

struct Metrics {
    a: Counter,
    b: Gauge,
}

Code to register the metrics in Metrics with a Registry could be generated via a derive macro on Metrics.

• The metric name would be taken from the field name.
• The metric help text would be taken from the Rust doc String.
• The metric unit could be provided via a macro attribute.

#[derive(Register)]
struct Metrics {
    /// My Gauge tracking the number of xxx
    a: Counter,
    /// My Counter, tracking the event xxx
    #[prometheus-unit(Seconds)]
    b: Gauge,
}

Where the Register trait is something along the lines of:

trait Register {
  fn create_and_register(registry: &mut Registry) -> Self;
}

Start with the proposed Register trait and the derive example in the issue. Determine how field names, Rust doc strings, and the prometheus-unit attribute should map to metric registration, then define what generated create_and_register behavior must produce for the Metrics example.