Large prometheus scrape body uses a lot of memory

[yawkat]

Issue description

micronaut-micrometer/micrometer-registry-prometheus/src/main/java/io/micronaut/configuration/metrics/micrometer/prometheus/management/PrometheusEndpoint.java

Lines 48 to 50 in c61f956

	public String scrape() {
	return prometheusMeterRegistry.scrape();
	}

This code transforms the entire meter registry into a String for the scrape endpoint. If there are a lot of metrics, this can use a bunch of memory. Additionally, micronaut-http-server-netty allocates a buffer for the full string, which is slow for multi-MB strings.

It would be better to stream the response using the scrape(OutputStream) method. I don't think it's a straight-forward change though.