in processor/src/main/java/com/google/cloud/solutions/realtimedash/pipeline/MetricsCalculationPipeline.java [203:207]
public void buildHllKey(ProcessContext context) {
Pair<String, String> elem = context.element();
context.output(
KV.of("hll_" + buildPrefix(name) + elem.getKey(), elem.getValue()));
}