`apm.startTransaction()` should return a NoopTransaction if the Agent is not yet started

[trentm]

See discussion on #2425. It was decided there that, while the TypeScript types in index.d.ts allow apm.startTransaction() returning null is some cases, it could be a breaking change to JS users of apm.startTransaction(...) that did not handle this case -- especially because the Agent API docs for startTransaction don't mention the possibility of getting null.

Update: This comment has the best description for why we are dropping | null from the return type of apm.startTransaction().

[trentm]

Consider changing this plan to instead return a NoopTransaction (i.e. taking advice from what OTel does in the equivalent case). And then could remove | null from index.d.ts for startTransaction().

[trentm]

taking advice from what OTel does in the equivalent case

OTel API has a NonRecordingSpan.

export const INVALID_SPANID = '0000000000000000';
export const INVALID_TRACEID = '00000000000000000000000000000000';
export const INVALID_SPAN_CONTEXT: SpanContext = {
  traceId: INVALID_TRACEID,
  spanId: INVALID_SPANID,
  traceFlags: TraceFlags.NONE,
};

Proposal:

• NoopTransaction, uses all-zeros ids. Implements all of interface Transaction
• Agent.protototype.startTrannsaction returns NoopTransaction if agent is not yet started.
• remove | null from startTransaction type.
• add a test case that exercises a transaction created via the API without the agent started.

[trentm]

Closed by #3554 on the "dev/4.x" branch, which will be the active 4.x branch soonish.