### Description

From the nodes stats API, we should extract the `ingest` stats into their own endpoint. This endpoint can then be non-node-centric, and present a global view of the pipelines, processors, and the metrics collected around ingest in general.
server/src/main/java/org/elasticsearch/ingest/IngestStats.java
implements
            Writeable,
            ToXContentFragment {
        /**
         * Read from a stream.
         */
        public Stats(StreamInput in) throws IOException {
            this(in.readVLong(), in.readVLong(), in.readVLong(), in.readVLong());
        }
        @Override
        public void writeTo(StreamOutput out) throws IOException {
            out.writeVLong(ingestCount);
            out.writeVLong(ingestTimeInMillis);
            out.writeVLong(ingestCurrent);
            out.writeVLong(ingestFailedCount);
        }
        @Override
        public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
            builder.field("count", ingestCount);
            builder.humanReadableField("time_in_millis", "time", new TimeValue(ingestTimeInMillis, TimeUnit.MILLISECONDS));
            builder.field("current", ingestCurrent);
            builder.field("failed", ingestFailedCount);
            return builder;
        }
    }
    /**
     * Easy conversion from scoped {@link IngestMetric} objects to a serializable Stats objects
     */
    static class Builder {
        private Stats totalStats = null;