@Override
  public int sizeOnServer() {
    ServerRegionProxy proxy = getServerProxy();
    if (proxy != null) {
      return proxy.size();
    }
    throw new UnsupportedOperationException(
        "sizeOnServer requires a pool.");
  }
finds the keys in this region using the given interestType and argument. Currently only
InterestType.REGULAR_EXPRESSION and InterestType.KEY are supported

@param interestType an InterestType value
@param interestArg the associated argument (regex string, key or key list, etc)
@param allowTombstones whether to return destroyed entries
@return a set of the keys matching the given criterion
geode-core/src/main/java/org/apache/geode/internal/cache/LocalRegion.java