[Improvement] [Open Telemetry] Add db.operation attribute

[yahorsi]

Currently, the trace attributes emmited by StackExchange.Redis there is only db.statement attribute that for many commands contains the full command with its parameters, which makes it close to impossible to group by the operation.

So the request it to add a new attribute - db.operation that will only contain the operation without the parameters. This will make it easier to analyse traces by the grouping by the operation.

PS: PHP Open Telemetry SDK emmits both db.statement and db.operation

[NickCraver]

While we plan to heads down at some point on a lot of OTel, I think there's confusion here: we don't emit db.statement today, that's something not in the core library you're getting now.