Add KeyValue support for Observation.Event

[lenin-jaganathan]

Please describe the feature request. Currently, Observation.Event supports only providing a name (and contextual name). The enhancement request is to allow the addition of low cardinality and high cardinality tags (aka attributes) to events.

Rationale This would allow the Observation handlers to use them appropriately for use cases like tagging the counters created from the events with more informative tags, key value-based spans/logging systems can use this to enhance search capabilities and enrich events with additional contextual information, etc. Adding support for tags in events would also enable places where certain tags are associated with an event to be flushed immediately and remove references. Today, such information can only be added to Observation which might be long-lived. Also, we cannot add too many low cardinal tags to the observation as that would make it emit timers with too many attributes. Doing so in events would ease these off from timers and add to counters which are relatively cheaper compared to Timers.