Rename `db.operation` to `db.operation.name` for future proofing

[trask]

This will give us the chance to add more attributes under the db.operation.* namespace post-stabilization.

In particular, @lmolkova's #875 (comment):

I think we might entertain the idea of db.operation.duration metric (and it might still be an open question for messaging metrics since operation list is an open set).

In case of databases, we have a precedent when CosmosDB records operation name (e.g. ReadItems) along with db.cosmosdb.operation_type (e.g. Read). We might want to generalize at some point.
If we translated it to messaging, we'd do something like messaging.operation.type = settle, messaging.operation.name = ack.

and #875 (comment):

Also, graphql has operation.name and operation.type - https://github.com/open-telemetry/semantic-conventions/blob/main/docs/graphql/graphql-spans.md

[arminru]

Done in #875.