add Counter/Gauge/Timer/Histogram traits to core zio-metrics jar

[pjfanning]

• https://github.com/zio/zio-metrics/blob/zio2/dropwizard/src/main/scala/zio/metrics/dropwizard/Metrics.scala
• https://github.com/zio/zio-metrics/blob/zio2/prometheus/src/main/scala/zio/metrics/prometheus/Metrics.scala

These dropwizard and prometheus metrics have very similar APIs but they don't share a super class (or interface/trait). From an object oriented programming perspective, it would be nice to have a shared Counter trait (as one example).

trait CounterMetric {
  def inc(): Task[Unit]
  def inc(amount: Double): Task[Unit]
}