[APM] Document infra plugin is required to use APM

[alisonelizabeth]

Kibana version: 7.13.4

Setting xpack.infra.enabled: false in kibana.yml disables APM UI in Kibana.

Per the docs, it appears the UI is controlled by xpack.apm.enabled. It would be helpful to update the docs to also note that disabling the infra plugin will also disable the APM UI (assuming this is the correct behavior).

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)

[sorenlouv]

I'm not sure if this is something we should document. APM currently require 9 plugins. Should we document them too?

kibana/x-pack/plugins/apm/kibana.json

Lines 9 to 19 in a49700f

	"requiredPlugins": [
	"apmOss",
	"data",
	"embeddable",
	"features",
	"infra",
	"licensing",
	"observability",
	"ruleRegistry",
	"triggersActionsUi"
	],

In the future far fewer plugins will have the ability to be disabled making this less of a problem.

@bmorelli25 WDYT? Should we document this?

[bmorelli25]

I'm also tempted to leave this out of the docs. It looks like the required plugin list changes somewhat frequently, so we would need to continuously update the list. From my searching, this seems to be the first time a user has reported this problem to us; if it happens again, then I say we doc.

[alisonelizabeth]

👍 works for me. FWIW I believe the user who encountered this may have unintentionally disabled the infra plugin so this is probably an edge case.

[sorenlouv]

Related: By default, a plugin should not be disable-able