This PR introduces changes to the MetaDataHandler class in the venice-router component to add a metric to track client D2 discovery of stores. Additionally, it updates the corresponding test class to accommodate these changes.

Resolves #1407

How was this PR tested?

Tested in CI and successfully ran these test locally:

gradlew clean -DmaxParallelForks=2  --parallel -x :internal:venice-avro-compatibility-test:test jacocoTestCoverageVerification diffCoverage --continue
gradlew --continue --no-daemon -DmaxParallelForks=2 --parallel -Pspotallbugs check -x spotlessCheck -x test -x integrationTest -x jacocoTestCoverageVerification

Does this PR introduce any user-facing changes?

• No. You can skip the rest of this section.
• Yes. Make sure to explain your proposed changes and call out the behavior change.