Rename db.client.connections.usage to db.client.connections.count?

[trask]

This seems more consistent with:

• system.processes.count

(though also could be db.client.connections based on outcome of #50)

[trask]

Examples of .count:

• process.runtime.jvm.threads.count
• system.processes.count

Examples of .usage:

• db.client.connections.usage
• system.filesystem.usage (Bytes)
• system.memory.usage (Bytes)
• system.paging.usage (Bytes)
• process.runtime.jvm.memory.usage (Bytes)
• process.runtime.jvm.memory.usage_after_last_gc (Bytes)
• hw.gpu.memory.usage (Bytes)
• hw.logical_disk.usage (Bytes)

Proposal: recommend .usage when units are Bytes, otherwise recommend .count

[lmolkova]

Related: #703 (maybe client.connection) ?

[trask]

let's do this change along with initial stability even though we aren't targeting stability for this metric

[gregkalapos]

I'd be happy to create a PR for this.