Public encoding functions?

Sometimes I have already e.g. nodes: Vec<Node> in the application state where Node is like

struct Node {
   url: String,
   requests: AtomicU64,
}

(When you are already working with a &Node, bumping the requests counter is just an atomic inc. It's much more efficient than get_or_create(...).inc().)

And I'd like to expose the requests counter of all nodes as a metric family. It might be possible with the collector API, but it would still require quite some boxing, cloning and iterator mapping.

Instead I'd like to encode the metrics myself, e.g.:

encode_descriptor(&mut buf, ...);
for n in &ctx.nodes {
    encode_counter_with_labels(&mut buf, [("node", &n.url)], n.requests.load(...));
}
encode_eof(&mut buf);

This will not require any extra allocation or cloning. By fusing the collecting and encoding phase, metrics can be encoded very efficiently.

Start by reviewing the collector API and the existing metric encoding paths. Determine how public descriptor, labeled-counter, and end-of-output functions could support collecting and encoding in one pass without the allocations described in the issue. Done means the proposed public API is defined and its behavior is covered by the relevant tests.