All credits on the initial idea, implementation, and testing belong to
@amunra, who is the original author of this PR prometheus#154.

From the original PR description:

Adds new `encode_registry` and `encode_eof` functions to allow encoding
of parts of the response.

This is useful when there are multiple registries at play, or when
composing metrics for a process that embeds Rust as part of its logic
whilst serving metrics to Prometheus outside of Rust.

Fixes: prometheus#153
Refs: prometheus#154, prometheus#204

Co-authored-by: amunra <cimarosti@gmail.com>
Signed-off-by: tyrone-wu <wudevelops@gmail.com>

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

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