[Tracing Appender] Propagate event name to exporters

[lalitb]

As per the recent changes in Tokio Tracing library - tokio-rs/tracing#2699, it now allows setting event name in tracing macros. E.g., in

error!(name: "my-event-name", event_id = 20, user_name = "otel", user_email = "otel@opentelemetry.io");

As lots of backends support event/log name (ElasticSearch, Fluentd, Sentry etc), it should be possible for users to specify event name using tracing macros, and export them to these backend through custom otel-exporters.

The PR modifies tracing subscriber to propagates this event-name to exporters by adding it to attributes. And if the attributes already have the key with the same name, that would override the "name" metadata field. Also, modified user_events log exporter to utilize this field. As of now, user_events exporter expect user to provide this field as "event_name" attribute through tracing macros.

TBD - It is also good to have parity across the metadata field in tracing event, and the log-record, at-least log-record structure should optionally have the additive field from the event metadata (and name is one such field). I plan to subsequently propose adding this field in LogRecord implementation separately.

[codecov]

Codecov Report

Attention: 13 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
opentelemetry-appender-tracing/src/layer.rs	0.0% <0.0%> (ø)
...pentelemetry-user-events-logs/src/logs/exporter.rs	0.0% <0.0%> (ø)

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[cijothomas]

Please add changelogs too!