MINOR: Fix the way total consumed is calculated for verifiable consumer

[skaundinya15]

Currently the way we calculate the number of total consumed messages for the verifiable consumer overcounts the number of actually consumed messages. This PR is to fix that to ensure we count the number of consumed messages correctly.

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)

[rondagostino]

Wow, nice catch. LGTM.

[rajinisivaram]

@skaundinya15 Thanks for the PR, LGTM

[skaundinya15]

Link for system test run for this branch: http://confluent-kafka-branch-builder-system-test-results.s3-us-west-2.amazonaws.com/2020-08-07--001.1596860596--skaundinya15--minor-fix-total-consumed-for-verifiable-consumer--d799e563e/report.html. The one test that might be related is streams_broker_compatibility_test but it looked like it failed on something unrelated, so I am rerunning that test to see if it it's okay.

[skaundinya15]

I re-ran the system tests for streams_broker_compatibility_test here: http://confluent-kafka-branch-builder-system-test-results.s3-us-west-2.amazonaws.com/2020-08-10--001.1597088498--skaundinya15--minor-fix-total-consumed-for-verifiable-consumer--d799e563e/report.html and got a green build. Seems like anything dependent on this method/variable checks out ok for this.

[rajinisivaram]

@skaundinya15 Thanks for running the tests, merging to trunk.