performance optimization: concrete impls for NumericMessage

[fommil]

We've noticed an excessive number of Double boxings when recording histogram metrics. They land in https://github.com/DataDog/java-dogstatsd-client/blob/master/src/main/java/com/timgroup/statsd/NonBlockingStatsDClient.java#L522

which is a generic version of https://github.com/DataDog/java-dogstatsd-client/blob/master/src/main/java/com/timgroup/statsd/NumericMessage.java

The value is always boxed as a result of the Double.of call.

It would greatly reduce object allocations if there was a concrete non-generic implementation of NumericMessage for each of the types it supports (int, long and double), and then used by the histogram feature. Arguably just long and double impls should be enough.

[fommil]

example Double impl. NOTE the subtle change to use .get().format when writing out primitive values, to avoid a box.

    class StatsDDoubleMessage extends Message {
        final double sampleRate; // NaN for none
        final long timestamp; // zero for none
        double value;

        StatsDDoubleMessage(
                            String aspect,
                            Message.Type type,
                            double value,
                            double sampleRate,
                            long timestamp,
                            TagsCardinality card,
                            String[] tags) {
            super(aspect, type, card, tags);
            this.value = value;
            this.sampleRate = sampleRate;
            this.timestamp = timestamp;
        }

        @Override
        public final boolean writeTo(StringBuilder builder, int capacity) {
            builder.append(prefix).append(aspect).append(':');
            writeValue(builder);
            builder.append('|').append(type);
            if (!Double.isNaN(sampleRate)) {
                builder.append('|').append('@').append(SAMPLE_RATE_FORMATTER.get().format(sampleRate));
            }
            if (timestamp != 0) {
                builder.append("|T").append(timestamp);
            }
            tagString(this.tags, builder);
            writeMessageTail(builder, tagsCardinality);
            return false;
        }

        @Override
        public boolean canAggregate() {
            // Timestamped values can not be aggregated.
            return super.canAggregate() && this.timestamp == 0;
        }

        protected void writeValue(StringBuilder builder) {
            builder.append(NUMBER_FORMATTER.get().format(this.value));
        }

        @Override
        public void aggregate(Message message) {
            var msg = (StatsDDoubleMessage) message;
            double value = msg.getValue();
            switch (msg.getType()) {
            case GAUGE:
                setValue(value);
                break;
            default:
                setValue(getValue() + value);
            }
            return;
        }

        public void setValue(double value) {
            this.value = value;
        }
        public double getValue() {
            return this.value;
        }
    }