Document *.client.{system}.* ordering in the metric naming

[lmolkova]

We need to decide on a pattern for system-specific client/server metrics.

The pattern for client/server is {domain}.client.metric_name and {domain}.server.metric_name (e.g. http.client.request.duration and http.server.request.duration).

For db/messaging/rpc/genai/etc, we'll need to take into account system-specific metrics and have a consistent place for the system identifier.

E.g.

1. db.cosmosdb.client.* and db.cosmosdb.server.*
   vs
2. db.client.cosmosdb.* and db.server.cosmosdb.*

Option 1 (*.{system}.client.*) is aligned better with attribute naming pattern ({domain}.{system}.attribute_name).
Option 2 (*.client.{system}.*) is aligned better with metric naming pattern ({domain}.client.{system}.metric_name).

There is no obvious advantage of one options versus the other, but keeping things consistent is important. Let's pick one option and document it as a guidance.

[lmolkova]

I have a slight preference for Option 2 (since we're picking a metric name pattern, it's more important to align with metric names than attribute names).

I.e. we'd have db.client.operation.duration and db.client.cosmosdb.metric_name.