Adopt encoding style similar to serde. `EncodeXXX` for a type that can be
encoded (see serde's `Serialize`) and `XXXEncoder` for a supported encoding i.e.
text and protobuf (see serde's `Serializer`).

- Compatible with Rust's additive features. Enabling the `protobuf` feature does
not change type signatures.
- `EncodeMetric` is trait object safe, and thus `Registry` can use dynamic dispatch.
- Implement a single encoding trait `EncodeMetric` per metric type instead of
one implementation per metric AND per encoding format.
- Leverage `std::fmt::Write` instead of `std::io::Write`. The OpenMetrics text
format has to be unicode compliant. The OpenMetrics Protobuf format requires
labels to be valid strings.

Signed-off-by: Max Inden <mail@max-inden.de>

Signed-off-by: Max Inden <mail@max-inden.de>

Signed-off-by: Max Inden <mail@max-inden.de>

The `Collector` abstraction allows users to provide additional metrics
and their description on each scrape.

See also:

- https://pkg.go.dev/github.com/prometheus/client_golang/prometheus#hdr-Custom_Collectors_and_constant_Metrics
- prometheus#49
- prometheus#29

Signed-off-by: Max Inden <mail@max-inden.de>

Signed-off-by: Max Inden <mail@max-inden.de>

Iterators returned by a Collector don't have to cross thread boundaries, nor doe
their references.

Signed-off-by: Max Inden <mail@max-inden.de>

Signed-off-by: Max Inden <mail@max-inden.de>

…to collector

Signed-off-by: Max Inden <mail@max-inden.de>

Signed-off-by: Max Inden <mail@max-inden.de>

Signed-off-by: Max Inden <mail@max-inden.de>