[server] Adding metric to capture key not found

[ymuppala]

Problem Statement

The Venice storage nodes (server-side) lack visibility into the rate of "key not found" events (empty or null responses).

Solution

This PR introduces new server-side metrics to track "key not found" events (empty/null responses) across all read request types (Single Get, Multi-Get, and Compute). These metrics provide visibility into the rate of missing keys at both the per-store and host-aggregate levels, helping identify scenarios where high read traffic results in lookups for non-existent data.

Code changes

• Added new code behind a config. If so list the config names and their default values in the PR description.
• Introduced new log lines.
  • Confirmed if logs need to be rate limited to avoid excessive logging.

Concurrency-Specific Checks

Both reviewer and PR author to verify

• Code has no race conditions or thread safety issues.
• Proper synchronization mechanisms (e.g., synchronized, RWLock) are used where needed.
• No blocking calls inside critical sections that could lead to deadlocks or performance degradation.
• Verified thread-safe collections are used (e.g., ConcurrentHashMap, CopyOnWriteArrayList).
• Validated proper exception handling in multi-threaded code to avoid silent thread termination.

How was this PR tested?

• New unit tests added.
• New integration tests added.
• Modified or extended existing tests.
• Verified backward compatibility (if applicable).

Does this PR introduce any user-facing or breaking changes?

• No. You can skip the rest of this section.
• Yes. Clearly explain the behavior change and its impact.

[m-nagarajan]

LGTM, thanks