Store well-defined agent-produced metrics in shared metrics data stream

[axw]

Currently, builtin metrics sent by all agents like system.memory.total are stored in the data stream metrics-apm.app.<service>-<namespace>. This has the effect of creating many data streams, and hence shards, by default.

We should only create service-specific data streams where there is some risk of mapping conflicts, i.e. for custom application metrics. All others should be stored in shared data streams: either the existing metrics-apm.internal-<namespace>, or a new one.