Enhance metrics doc and Javadoc on Helidon metrics API types #7030

Job	Run time
build	5m 58s
copyright	3m 28s
shellcheck	3m 10s
checkstyle	3m 51s
tests/tck-cdi	17m 40s
tests/core	11m 12s
spotbugs/core	10m 40s
tests/tck-rest	14m 33s
tests/it	14m 42s
tests/native-image-mp-1-linux	6m 53s
spotbugs/integrations	7m 18s
tests/dbclient	9m 3s
tests/tck-others	16m 57s
tests/native-image-mp-2-linux	12m 8s
spotbugs/others	12m 57s
tests/dbclient-oracle	17m 7s
tests/native-image-mp-3-linux	6m 51s
tests/others	13m 9s
tests/native-image-se-1-linux	6m 53s
archetypes/r1-jar	7m 8s
tests/native-image-inject-linux	6m 24s
docs	4m 56s
archetypes/r2-jar	6m 51s
examples/linux	12m 54s
tests/native-image-mp-1-macos	8m 28s
archetypes/r3-jar	6m 17s
examples/macos	9m 47s
tests/native-image-mp-2-macos	15m 26s
javadoc (core)	7m 14s
archetypes/r4-jar	7m 9s
tests/native-image-mp-3-macos	9m 28s
javadoc (integrations)	5m 32s
archetypes/r5-jar	5m 30s
archetypes/legacy	4m 38s
tests/native-image-se-1-macos	6m 37s
javadoc (others)	7m 48s
tests/native-image-inject-macos	7m 33s
tests/packaging-jar-linux	7m 57s
quickstarts/linux	12m 7s
tests/packaging-jlink-linux	8m 53s
quickstarts/macos	12m 55s
tests/packaging-jar-macos	4m 55s
tests/packaging-jlink-macos	6m 43s
tests/results	14s
gate	3s
	6h 27m 57s

Provide feedback