grpc: change the LB policy config parsing API to accept JSON values instead of String

[easwars]

The existing parse_config method on the LbPolicyBuilder trait accepts configuration as a &str. Passing parsed JSON to this API is better for the following reasons:

• the LB policy configuration is part of the ServiceConfig which is also represented as JSON. The channel would parse the service config before even attempting to send the LB policy configuration portion of the ServiceConfig to the LB policy builder. Therefore, the LB policy configuration is already available as parsed JSON.
• Aligns better with other gRPC languages (except Go).

[easwars]

@dfawley @LucioFranco

[dfawley]

Suggested change

	pub struct ParsedJsonLbConfig(pub serde_json::Value);
	pub struct ParsedJsonLbConfig(serde_json::Value);

The idea is to encapsulate serde_json and not expose it outside our crate.

[easwars]

Currently the channel (in the gracefulswitch LB policy) specifies a default service config (with pick_first as the LB policy). So, we have a few options:

• Make this field pub(super) so that the channel has access to this field. Also, maybe the channel would want this when it implements the service config parsing from the resolver?
• Make this field pub(crate) if other tests end wanting to set a service config.
• Have an associated function new that takes the input JSON as a string, and returns Result<Self, String>.
  • we could use the serde_json::from_str function to convert the input string into a serde_json::Value

What do you think?

[LucioFranco]

New function makes the most sense or you can even implement FromStr, exposing the internal type to modules outside of this file is generally a bad idea. (eg you want to add a field to this struct then you need to refactor all the locations that reference the internals.

[easwars]

Moved the value to be a struct field instead of this being a tuple struct.

Also, added a new function.

[easwars]

Gentle ping on this PR: @LucioFranco @dfawley