Introduce X-Webrpc-Client header to all generated clients

[VojtechVitek]

I propose to add a new X-Webrpc-Client header to all webrpc client requests:

X-Webrpc-Client: {serviceName}@{schemaVersion}; webrpc@{webrpcGenVersion}; github.com/webrpc/gen-golang@{templateVersion}

Example:

X-Webrpc-Client: indexer@v0.2.0; webrpc@v0.18.0; github.com/webrpc/gen-golang@v0.18.2

Potential use-cases from the server perspective:

• Telemetry on client version distribution
• Enables server to error out on "deprecated" version of the schema
• Very helpful for debugging HTTP 400 errors (missing fields, renamed fields etc.)

Go server

Let's add a helper function to help parse the schema version easily.

func GetClientHeader(r *http.Request) (schema string, webrpc string, gen string) {
  // value of header or ""
}

func ParseVersion(r *http.Request) (major, minor, bugfix int) {
  // parse X-Webrpc-Client header and return the details of the schema version
  // returns 0, 0, 0 if header is missing
}

func (s *ExampleServiceRPC) GetUser(ctx context.Context, header map[string]string, userID uint64) (*User, error) {
	schema, webrpc, gen := proto.GetClientHeader(r)
	
	major, minor, _ := proto.ParseVersion(schema)
	if major <= 1 && minor <= 8 {
		return nil, proto.ErrUnsupportedClientVersion.WithCause(fmt.Errorf("the latest supported webrpc client version is 1.8.x"))
	}
	
	// ...
}

BREAKING CHANGE?

Adding a new header to all requests might lead to CORS errors unless the server whitelists the new header. So perhaps we should make this an opt-in feature for now. However, I'd greatly support making this field mandatory as part of "webrpc protocol standard" down the road. Maybe we can make this an opt-out feature after a few releases. Or maybe we can enforce it in the generated client initialization - a new required field when you instantiate the client. Thoughts?

[david-littlefarmer]

Two thoughts

First

I tried to look up the naming convention for headers, and it looks like, the use of X- prefix is not recommended anymore.
Check this https://www.rfc-editor.org/rfc/rfc6648
This can be ignored it and just use X-Webrpc-Client instead of Webrpc-Client.

Second

I would change the structure from this:
X-Webrpc-Client: {serviceName}@{schemaVersion}; webrpc@{webrpcGenVersion}; github.com/webrpc/gen-golang@{templateVersion}

To this:
X-Webrpc-Client: service={serviceName}@{schemaVersion}; webrpc-verison={webrpcGenVersion}; template={template-source}@{templateVersion}

So the change would be to make a key-val string, same as for Set-Cookie https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Set-Cookie

So for example:
X-Webrpc-Client: service=indexer@v0.2.0; webrpc-version=v0.18.0; template=github.com/webrpc/gen-golang@v0.18.2

===

And to combine both thoughts:
Webrpc-Client: service=indexer@v0.2.0; webrpc-version=v0.18.0; template=github.com/webrpc/gen-golang@v0.18.2