code: public static Measurement first(final Iterable<Measurement> ms, final String k, final String v) {
      return first(ms, value -> v.equals(getTagValue(value.id(), k)));
    }
docs: Returns the first measurement with a given tag value.
  
  @param ms
  A set of measurements.
  @param k
  Key to search for.
  @param v
  Value that should be associated with k on the ids.
  @return
  Measurement or null if no matches are found.