[Enhancement] Modify variable names to enhance readability

[Stephanie0002]

Before Creating the Enhancement Request

• I have confirmed that this should be classified as an enhancement rather than a bug/feature.

Summary

Modify the variable names subscription in file ./example/src/main/java/org/apache/rocketmq/example/operation/Consumer.java to enhance readability

Motivation

Subscription relationships are generally defined using subscription groups. Create different subscription groups, and different consumers determine their subscription relationships to topics by belonging to different subscription groups. Therefore, the variable name ‘group’ in this file not only contains the meaning of the group, but also the meaning of the subscription relationship. On the other hand, the file defines the filtering rule expression strings of different consumers in subscription relationships as variable ‘subscription’, but the meaning of this variable name mainly lies in the filtering rule expression. The ambiguity in the definitions of these two variable names can easily lead people to believe that variable ‘group’ only expresses consumer groups, The variable ‘subscription’ represents a subscription relationship.

Describe the Solution You'd Like

I plan to change the variable name 'group' to 'subGroup' and change the variable name 'subscription' to 'subExpression'

Describe Alternatives You've Considered

change the variable name 'group' to 'subGroup' and change the variable name 'subscription' to 'subFilterExpression'

Additional Context

No response