Add an overloaded metricOrElseCreate function in Metrics

The `Metrics` registry is often used by concurrent threads, however it's get/create APIs are not well suited for it. A common pattern from the user today is:

metric = metrics.metric(metricName);

if (metric == null) {
  try {
    metrics.createMetric(..)
  } catch (IllegalArgumentException e){
    // another thread may create the metric at the mean time
  }
} 

Otherwise the caller would need to synchronize the whole block trying to get the metric. However, the `createMetric` function call itself indeed synchronize internally on updating the metric map.

So we could consider adding a metricOrElseCreate function which is similar to createMetric, but instead of throwing an illegal argument exception within the internal synchronization block, it would just return the already existing metric.