feat: add test and coverage test for openmldb-exporter

**Describe the feature you'd like**

[openmldb-exporter](https://github.com/4paradigm/OpenMLDB/tree/main/monitoring) is the prometheus exporter for OpenMLDB. It lacks unit tests and coverage reports

**Additional context**

Add any other context or screenshots about the feature request here.

The openmldb-exporter is in the monitoring directory. First, examine its existing code to understand what metrics it exposes and how it interacts with OpenMLDB. Look for existing test patterns in the repository to follow. Write unit tests for the exporter's functions and integrate a coverage tool like go test -cover. Ensure tests run successfully and generate a coverage report.

索引モデルが issue の本文から書いたものです。