[Feature Request] SDK connections should enable keep-alive by default and allow keep-alive customization

[bergundy]

Due to the number of issues we've gotten where idle clients take a long time to discover when a TCP connection is closed we decided to turn keep-alive on by default. We also should allow users to customize it if they can't today.

Links:

• https://github.com/grpc/grpc/blob/master/doc/keepalive.md
• Connection management issues with Java SDK sdk-java#362 (comment)
• Make keep alive configurable and opt-in sdk-go#371 (Go SDK issue where this was made opt-in)

SDK issues:

• TS - feat(client): Enable gRPC keep-alive by default sdk-typescript#855 (TODO: Expose some of these options to users?)
• Core - [Feature Request] Add client keep-alive support sdk-core#585
• TS worker - [Feature Request] Add NativeClient keep alive support sdk-typescript#1275
• Python - [Feature Request] Client keep alive support sdk-python#395
• .NET - [Feature Request] Support client keep alive sdk-dotnet#125
• Go - Enable TCP Keep alive by default sdk-go#1248
• Java - Enable TCP Keep alive by default sdk-java#1872
• PHP

[bergundy]

Setting the default to:

      'grpc.keepalive_permit_without_calls': 1,
      'grpc.keepalive_time_ms': 30_000,
      'grpc.keepalive_timeout_ms': 15_000,

Based on the previous Go SDK default.