Kibana APM user capture setting

[graphaelli]

Describe the feature:

Add opt-in for capturing user data in Kibana's own APM. Other apm settings live under elastic.apm like elastic.apm.enabled. I propose elastic.apm.capture_user with a default of false.

Describe a specific use case for the feature:

User capture is filtered out unconditionally in:

kibana/src/apm.js

Lines 37 to 48 in 1d49166

	// Filter out all user PII
	apm.addFilter((payload) => {
	try {
	if (payload.context && payload.context.user && typeof payload.context.user === 'object') {
	Object.keys(payload.context.user).forEach((key) => {
	payload.context.user[key] = '[REDACTED]';
	});
	}
	} finally {
	return payload;
	}
	});

elastic/apm#272 proposes to add a setting for this. This request should not be blocked by that but could utilize the setting if the relevant changes land in the nodejs agent before this is picked up.

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)

[pgayvallet]

Addressed by #176566