stop setting `span.id`, set `transaction.id` for log correlation

[trentm]

I noticed in elastic/apm-agent-nodejs#3399 discussion that, while the ecs-logging-nodejs libs are currently setting span.id or transaction.id for log-correlation, the ecs-logging spec (https://github.com/elastic/ecs-logging/blob/main/spec/spec.json) only mentions setting transaction.id. So it appears these libs have this wrong.

TODO:

• Confirm whether log-correlation works for log records that set trace.id and span.id. I.e. are these libs breaking log correlation for some log records?
• Change to conform to the spec (and what the Java APM agent is doing).

[bplies-ATX]

The Elastic Common Schema's Tracing fields define them.

The Node APM Agent docs only casually mention Spans

In one service, start a transaction with apm.startTransaction(), or a span with apm.startSpan().

The W3C traceparent specification describe "span-id" as also a "parent-id".

3.2.2.2 version-format
The following version-format definition is used for version 00.

version-format = trace-id "-" parent-id "-" trace-flags
trace-id = 32HEXDIGLC ; 16 bytes array identifier. All zeroes forbidden
parent-id = 16HEXDIGLC ; 8 bytes array identifier. All zeroes forbidden
trace-flags = 2HEXDIGLC ; 8 bit flags. Currently, only one bit is used. See below for details

3.2.2.4 parent-id
This is the ID of this request as known by the caller (in some tracing systems, this is known as the span-id, where a span is the execution of a client request).

In our usage we both expect and see span.id so I presume the referenced schema is merely incomplete.