Map time units to UCUM format for Dynatrace

[jonatan-ivanov]

Reported by @pirgeo.

MeterRegistry implementations mapping TimeUnits to a string representation using its .toString() through this cache:

micrometer/micrometer-core/src/main/java/io/micrometer/core/instrument/MeterRegistry.java

Lines 70 to 78 in eff8a61

	private static final EnumMap<TimeUnit, String> BASE_TIME_UNIT_STRING_CACHE = Arrays.stream(TimeUnit.values())
	.collect(
	Collectors.toMap(
	Function.identity(),
	(timeUnit) -> timeUnit.toString().toLowerCase(),
	(k, v) -> { throw new IllegalStateException("Duplicate keys should not exist."); },
	() -> new EnumMap<>(TimeUnit.class)
	)
	);

This is the standard OpenJDK representation for time units but for Dynatrace, we need UCUM-compliant units, e.g.: s instead of seconds, ms instead of milliseconds, etc.