model/profile: deal properly with dynamic metric names

[axw]

Currently we will record any pprof sample values, but we rename the unit "nanoseconds" to "ns". However, the index mapping is not dynamic, and we only define fields for a well-known set of metrics:

• profile.cpu.ns
• profile.samples.count
• profile.alloc_objects.count
• profile.alloc_space.bytes
• profile.inuse_objects.count
• profile.inuse_space.bytes

We should either limit to indexing these metrics, or we should move them under a sub-group and make that dynamic.

[axw]

I'll resurrect the issue if/when we come back to this.