election: improve lease renewal observability, stability, and performance

## Enhancement Task

PD lease renewal is currently tied too closely to the configured lease TTL in some paths. When the lease TTL is increased, the `KeepAliveOnce` cadence can also become slower, which works against the goal of keeping leadership leases from expiring under runtime stalls, slow scheduling, or transient etcd latency.

Track lease renewal improvements around:

- Observability: expose and refine metrics/logging for renewal latency, response interval, remaining TTL, and termination reasons.
- Stability: make renewal cadence less sensitive to large lease TTL values and reduce lease-expiration risk.
- Performance: keep the renewal loop lightweight and avoid unnecessary load while preserving a safe renewal frequency.

Initial scoped PR:

- Use a fixed upper bound for PD election lease `KeepAliveOnce` intervals so increasing the lease TTL does not proportionally slow renewal.