Revamp how the engine handles metadata

[jklaise]

Currently the engine merges the metadata between components based on whatever metadata the new component adds:

seldon-core/engine/src/main/java/io/seldon/engine/predictors/PredictiveUnitBean.java

Lines 350 to 364 in cbbcc6b

	private SeldonMessage mergeMeta(SeldonMessage message, List<SeldonMessage> messages) {
	Meta.Builder metaBuilder = Meta.newBuilder(message.getMeta());
	for (SeldonMessage originalMessage : messages){
	metaBuilder.putAllTags(originalMessage.getMeta().getTagsMap());
	}
	metaBuilder.clearMetrics();
	return SeldonMessage.newBuilder(message).setMeta(metaBuilder).build();
	}
	private SeldonMessage mergeMeta(SeldonMessage message, Meta meta) {
	Meta.Builder metaBuilder = Meta.newBuilder(message.getMeta());
	metaBuilder.putAllTags(meta.getTagsMap());
	metaBuilder.clearMetrics();
	return SeldonMessage.newBuilder(message).setMeta(metaBuilder).build();
	}

A drawback of this is that some metadata can get lost, e.g. the engine currently doesn't pass the puid along (#424 fixes this). A better way would be only to add to the metadata so nothing is lost in the flow.

[ukclivecox]

Close. Can reopen if we decide to revamp how metadata is handled in general on a per-request basis.