[Monitoring] Correctly type kibana config

[neptunian]

Currently the config has a getter that we use and pass in the string of the property config name, but it's typed to always return a string or undefined which is not always the case.

[elasticmachine]

Pinging @elastic/logs-metrics-ui (Team:logs-metrics-ui)

[matschaffer]

So I was looking at how infra plugin handles config (cc @weltenwort to confirm that this is a reasonable plugin to copy from) and it's quite different.

I suspect in order to get good typing of how monitoring plugin handles configs, we'll have to switch away from the config.get API in favor of accessing the config object directly, like on

kibana/x-pack/plugins/infra/server/lib/alerting/inventory_metric_threshold/inventory_metric_threshold_executor.ts

Line 109 in 270adf4

const compositeSize = libs.configuration.alerting.inventory_threshold.group_by_page_size;

[matschaffer]

I guess the other option might be to at least switch our config.get('key') calls to config.get<T>('key')

I'll try that out and see if it's worth the gain compared with the larger refactoring away from config.get

[matschaffer]

I think

kibana/x-pack/plugins/monitoring/server/plugin.ts

Lines 331 to 332 in 4bcee04

	if (key === 'server.uuid') {
	return this.legacyShimDependencies.instanceUuid;

eliminates the option of a generic since that will always return a string.

I'll see what happens if I start swapping out config.get for config: MonitoringConfig but I think the blast radius of the change will increase quite a bit so I'll increase the issue size to medium.