feat: Add readUnits, upsertedCount, and matchedRecords to ResponseMetadata

[rasharab]

BACKGROUND:
Our company is using your listener pattern for Otel metrics, but there's just not enough valuable information for us.
We're adding fields we consider necessary for our operations.

Adds usage and operation-result fields to ResponseMetadata so users can track read units, upserted count, and matched records via the metadata listener.

Changes:

• ResponseMetadata: Added three new nullable Integer fields (readUnits, upsertedCount, matchedRecords) with getters, builder methods, and conditional toString()
• ResponseMetadataInterceptor: Added onMessage() override to capture gRPC response bodies and extract fields via reflection. Added List to TRACKED_OPERATIONS.
• ResponseMetadataTest: 10 new tests (22 total, all passing)
• README.md: Updated observability example and method table with new getters

---

Note

Medium Risk
Medium risk because it changes the gRPC interceptor to reflectively parse response bodies during onMessage(), which could affect performance or subtly vary by response type; failures are handled by returning nulls with debug logging.

Overview
Adds richer per-request observability metadata by extending ResponseMetadata with three new nullable fields: readUnits, upsertedCount, and matchedRecords, including builder setters, getters, and toString() output only when present.

Enhances metadata capture in the gRPC client interceptor by tracking List operations and extracting the new fields from response messages via reflection during onMessage(), then emitting them through the existing response-metadata listener.

Updates README documentation/examples and expands unit test coverage to validate defaults, builder wiring, and toString() behavior for the new fields.

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[rasharab]

We use a LOT of list operations. This is necessary for us.

[rasharab]

FYI @jhamon