Increase code coverage

Currently the code coverage of pinot repo is just around 50% ([Link](https://codecov.io/github/apache/pinot)).
This issue tracks the status of increasing code coverage.

The following modules currently have less than 50% code coverage or don't have any test at all:
1. pinot-clients/pinot-java-client (46.15%)
2. pinot-common (47.96%)
3. pinot-plugins (36.38%)
No code coverage:
- pinot-plugins/pinot-batch-ingestion/pinot-batch-ingestion-common
- pinot-plugins/pinot-batch-ingestion/pinot-batch-ingestion-hadoop
- pinot-plugins/pinot-batch-ingestion/pinot-batch-ingestion-spark
- pinot-plugins/pinot-batch-ingestion/pinot-batch-ingestion-stadardalone
- pinot-plugins/pinot-file-system/pinot-gcs
- pinot-plugins/pinot-file-system/pinot-hdfs

The rest of the modules shown below seems fine but we should still bring them to at least 75%.
pinot-broker (76.6%)
pinot-controller (63.29%)
pinot-core (76.85%)
pinot-minion (68.98%)
pinot-server (67.05%)
pinot-spi (68.84%)

Review the Codecov report and the listed modules, starting with pinot-clients/pinot-java-client, pinot-common, and pinot-plugins. Identify modules without tests or below the stated coverage targets, then add coverage until the tracked modules reach at least 75% and verify the results in Codecov.

Written by the indexing model from the issue text.