feat: allow Arc<String> on EncodeLabelSet

[flaneur2020]

I'm writing a label struct like this:

#[derive(Debug, Clone, Hash, Eq, PartialEq, EncodeLabelSet)]
struct ErrorLabels {
    op: &'static str,
    scheme: &'static str,
    err: &'static str,
    root: Arc<String>,
    namespace: Arc<String>,
}

in this struct, the root and namespace field is nearly immutable after a client get established. i hope to use Arc to reduce copy Strings on updating metrics.

but i got this error:

the trait bound `for<'a> &'a std::string::String: EncodeLabelValue` is not satisfied
the trait `EncodeLabelValue` is implemented for `std::string::String`
required for `Arc<std::string::String>` to implement `EncodeLabelValue`

it tells that we need implement &String to EncodeLabelValue.

this PR addes a implement about EncodeLabelValue for &String, and it can make the previous labels struct compile successfully.

[mxinden]

Thanks!

Can you add a changelog entry in CHANGELOG.md?

Also, can you add a test in https://github.com/prometheus/client_rust/blob/master/derive-encode/tests/lib.rs

[flaneur2020]

sure, i'll add the test and changelog in this pr!

[mxinden]

Thank you!