Monitor weighted_tags

As early as possible in the producer right after the initial filter. And in the consumer when converting the UpdateEvent into an elastic update request. Ideally we'd prefer to monitor after the write but this requires to change again the AsyncElasticsearchBulkRequest just for the purpose of monitoring the tags. Another approach could have been to infer the tags from the elastic request itself but this sounded fragile. The assumption here is that we are close enough to the end of the pipeline that the metric should be accurate enough.

Bug: T373459