how to grep both files , but exclude other files

The simplest way to grep in two files, is to give both files to grep:

grep log.retention.bytes /confluent/logs/server.log /confluent/logs/server.log.1

When you have more files, you can either list them all manually or use a glob that matches only your specific files. For example, in bash:

shopt -s extglob
grep log.retention.bytes /confluent/logs/server.log?(.1)

Finally, and probably most simple in your case, you can use brace expansion as shown in Tomassz's answer.

grep log.retention.bytes server.log{,.1}

In order to keep log entries (appended) in chronological order, you might want to reverse the order of files:

grep log.retention.bytes server.log{.1,}

which is of course equivalent to:

grep log.retention.bytes server.log.1 server.log

as the brace expansion is done by the shell before executing the grep command.

Moreover, with zsh shell you can easily automatically glob for the last N files matching a pattern with:

grep log.retention.bytes server.log*(Om[-2,-1])

where Om means order by mtime descending and [-2,-1] fetches 2 last rows. This trick is worth remembering if you watch to search more files and do not want to type them manually.