Support extension data in emit_otlp

[KodrAus]

emit's data model is intentionally smaller than OpenTelemetry's. There are some features in OpenTelemetry that emit isn't planning any direct support for, including:

• span links
• span kinds

We could still let you define your own scheme for these and pull them off events in emit_otlp to better tailor your diagnostics. It could use the same closure-based approach we do for overriding the span body:

emit::setup()
    .emit_to(emit_otlp::new()
        .traces(emit_otlp::traces_http_proto("..")
            .span_kind(|evt| evt.props().get("span_kind"))
    ));

For span links, we could just define our own SpanLink type in emit_otlp and allow you to map properties on the event into span links.

[KodrAus]

One more extension that would be good to figure out would be histograms in metrics.

[KodrAus]

We can introduce extension data support without breaking changes so dropping this from 1.0.0.