wrapper to create a easy to use timegauge

TimeGauge.kt

import io.micrometer.core.instrument.MeterRegistry
import io.micrometer.core.instrument.Tag
import java.time.Instant
import java.util.concurrent.TimeUnit
import java.util.concurrent.atomic.AtomicLong
import java.util.function.ToDoubleFunction

fun MeterRegistry.timeGaugeMillis(name: String, tags: Iterable<Tag>, initialValue: Long = 0): (Instant) -> Unit {
    val a = AtomicLong(initialValue)
    a.apply {
        val deref = ToDoubleFunction<AtomicLong> { x -> x.get().toDouble() }
        [email protected]().timeGauge(name, tags, this, TimeUnit.MILLISECONDS, deref)
    }

    return { t: Instant -> a.set(t.toEpochMilli()) }
}