For elastic/apm#70, we have the need for top-level tags/labels for metrics. In particular, these are transaction.name, transaction.type, span.type and span.subtype. This is needed for a unified query bar experience. For example, filtering for a particular transaction.name should filter the related metrics as well as they should be used on the service detail view, powering the breakdown graphs.

Some ideas on how to enable top-level labels:

• APM Server knows about "well-known" tags like transaction_name and transaction_type and brings them to top-level
• Extend the metricset spec (https://github.com/elastic/apm-server/blob/master/docs/spec/metricsets/metricset.json) to allow for top-level key/value pairs (dots in the keys have to be allowed)