Add HTTP/2 keepalive options to ConnectionOptions

[h-michael]

Summary

• Add http2_keep_alive_interval, keep_alive_timeout, and keep_alive_while_idle fields to ConnectionOptions in gax, mapped to tonic Endpoint methods
• Expose these options through each gRPC service crate's config structs (spanner ChannelConfig, spanner admin AdminClientConfig, bigquery ChannelConfig, artifact-registry ClientConfig)
• pubsub and kms already expose ConnectionOptions directly, so no additional changes needed

Motivation

Without HTTP/2 keepalive, when an intermediate network device (load balancer, NAT, etc.) silently drops a connection, the client cannot detect the dead connection until the TCP retransmission timeout (~13-30
minutes depending on OS settings). This is particularly problematic for long-lived streaming RPCs like Pub/Sub streaming pull.

HTTP/2 PING frames allow the client to detect dead connections within interval + timeout (e.g., 50 seconds with 30s interval + 20s timeout) instead of waiting for TCP-level timeouts.

Google's official Go client configures HTTP/2 keepalive for this reason. tonic 0.14 already supports these settings on Endpoint without additional feature flags.

All defaults are None (keepalive disabled), so this is a non-breaking behavioral change. Users opt in by setting the fields explicitly.

Note: adding new fields to ConnectionOptions and other config structs is a source-breaking change for users who construct them with struct literals without ..Default::default().

Example

let config = ClientConfig {
    connection_option: ConnectionOptions {
        http2_keep_alive_interval: Some(Duration::from_secs(30)),
        keep_alive_timeout: Some(Duration::from_secs(20)),
        keep_alive_while_idle: Some(true),
        ..Default::default()
    },
    ..Default::default()
};

Note

@yoshidan
All new keepalive fields default to None (keepalive disabled) to preserve backward compatibility. An alternative would be to set sensible defaults (e.g., 30s interval, 20s timeout, idle=true) out of the box, which would benefit all users without requiring explicit configuration. Happy to change this if you prefer enabling keepalive by default.

Related issues

• Pub/Sub streaming pull connection stalls for minutes due to missing HTTP/2 keepalive configuration #429

[yoshidan]

LGTM.
We will initially release the update without changing the default values ​​to maintain backward compatibility.
Default values ​​may vary depending on the component.