gRPC Keepalive

Periodic HTTP/2 PING frames sent by gRPC to detect dead connections and keep NAT mappings alive, configurable via keepalive time, timeout, and permit without calls parameters. Without keepalives, idle connections may be silently dropped by firewalls or NAT devices, causing the next RPC to fail with a UNAVAILABLE error. The keepalive interval must be tuned to avoid being rejected as too aggressive by servers with strict rate limits on PINGs.