Add `dd.trace.db.client.split-by-instance` support for Redis

[zBart]

We use a couple of Redis clusters and right now and we would like to see requests on a per instance basis.
I noticed that currently this feature already exists for splitting DB requests per instance, but it doesn't work for Redis.

I took a quick look at the source code and I think it's because dbInstance returns null:

dd-trace-java/dd-java-agent/instrumentation/jedis-3.0/src/main/java/datadog/trace/instrumentation/jedis30/JedisClientDecorator.java

Line 48 in 6b96665

return null;

Maybe instead the hostname of the server can be passed instead?

[rr-paras-patel]

Another thing we don't have redis hostname attribute logged. so if app is connecting to multiple redis it's just blind assumption.