[SPARK-41115][CONNECT] Add ClientType to proto to indicate which client sends a request

[amaliujia]

What changes were proposed in this pull request?

This PRs introduces a ClientType into Connect proto that can be included into Request to indicate the client type (e.g. Python client, Scala client, etc.)

Why are the changes needed?

Better understand requests by the server.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

Existing UT

[amaliujia]

@grundprinzip @HyukjinKwon @zhengruifeng
cc @hvanhovell

[grundprinzip]

To avoid changing this all the time a new client comes along, this should most likely be a string.

Suggested change

	ClientType client_type = 4;
	optional string client_type = 4;

[grundprinzip]

Suggested change

	// From which client this request was sent from.
	// Provides optional information about the client sending the request. This field
	// can be used for language or version specific information and is only intended for
	// logging purposes and will not be interpreted by the server.

[grundprinzip]

Do you have a specific use case in mind?

[amaliujia]

@grundprinzip I actually want to log to different entries for the usage from different clients.

The perfect logging to me is

x jobs submitted through proto (which literally is the entire connect usage)
then breakdown
y from client A
z from client B.

Maybe, just say maybe, we will see user self-implemented clients by this field (or if this field is not set we will also know).

Also I would still imagine that we need send through client side language spec (language type, version, etc.) which you already mentioned in the suggestion. Maybe eventually we need a message named client metadata. But right now only which client is added to proto for this metadata.

We can see if we need client side language spec eventually (when we deal with UDF probably)

[grundprinzip]

All this in mind it still makes more sense to keep this as an optional string field instead of an enum.

[grundprinzip]

Please change the type from enum to string.

[amaliujia]

Yeah let me take a look.

[AmplabJenkins]

Can one of the admins verify this patch?

[zhengruifeng]

does this do the same thing as user_context.extensions ?

[grundprinzip]

No, this is purely a string and not an extension type based on Any

[grundprinzip]

@amaliujia can you please update the pr description to remove the enum part.

[HyukjinKwon]

Merged to master.