@Override
    public void close() {
        if (closed.getAndSet(true)) return;
        synchronized (nodeTable.writeLock) {
            synchronized (clusterTable.writeLock) {
                for (SqlCompiler sqlCompiler : sqlCompilerPool)
                    sqlCompiler.close();
                engine.close();
            }
        }
    }
An implementation of the metrics Db backed by Quest:
This provides local persistent storage of metrics with fast, multi-threaded lookup and write,
suitable for production.

@author bratseth
node-repository/src/main/java/com/yahoo/vespa/hosted/provision/autoscale/QuestMetricsDb.java