Support providing more buckets for histogram metrics

[davidpdrsn]

Fixes #8

Example usage:

Server::new(config)
    .with(routes)
    .metric_buckets(vec![(
        Matcher::Full("foobar".to_owned()),
        vec![0.0, 100.0, 200.0],
    )])
    .serve()
    .await;

Exactly what we have internally.