Add exponential backoff jitter to gRPC channel readiness

[dwkang]

Background

C++ agent gRPC reconnect behavior currently relies on fixed channel readiness waits. When collectors are down or restarting, many agents can retry on similar intervals and create a thundering herd risk.

The Java agent mitigates this with ExponentialBackoffReconnectJob using:

• initialInterval: 3000ms
• multiplier: 1.2
• randomizationFactor: 0.3
• maxInterval: 30000ms

Scope

• Add a reusable exponential backoff helper with jitter.
• Apply jittered backoff only in the shared gRPC channel readiness path.
• Use WaitForStateChange during the backoff window instead of sleeping, with 1s slices.
• Call readyChannel() before metadata unary RPCs.
• Reset backoff after the channel becomes ready.
• Add unit tests for delay growth, cap, reset, jitter bounds, and metadata channel-not-ready behavior.

Validation

• cmake --build --preset debug-cached
• ctest --preset debug-cached