feat(encoding): make decoder clone (#67)

## Description
All fields of the decoder are already clone, and I have a use case where
I'd like to clone the decoder.

## Breaking Changes
<!-- Optional, if there are any breaking changes document them,
including how to migrate older code. -->
## Notes & open questions
<!-- Any notes, remarks or open questions you have to make about the PR.
-->
## Change checklist
- [ ] Self-review.
- [ ] Documentation updates following the [style
guide](https://rust-lang.github.io/rfcs/1574-more-api-documentation-conventions.html#appendix-a-full-conventions-text),
if relevant.
- [ ] Tests if relevant.
- [ ] All breaking changes documented.

Author: b5

src/encoding.rs

@@ -230,7 +230,7 @@ impl Item<'_> {
 /// Decoder for metrics received from an [`Encoder`]
 ///
 /// Implements [`MetricsSource`] to export the decoded metrics to OpenMetrics.
-#[derive(Debug, Default)]
+#[derive(Debug, Clone, Default)]
 pub struct Decoder {
     schema: Option<Schema>,
     values: Values,