Expose serverBasePath on client-side

[joshdover]

Similar to #45991, Spaces needs access to the serverBasePath on the client-side. We should add this to the core.http.basePath interface in the browser.

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)

[pgayvallet]

What is the difference with http.basePath.get() ? Aren't client and server basePath the same?

[legrego]

As far as I can tell, http.basePath.get() on the client-side returns the space-aware base path. So if a user is currently in the marketing space, and the server base path is /foo, then http.basePath.get() returns /foo/s/marketing, instead of /foo.

I imagine we'll want to expose a dedicated serverBasePath property via http.basePath.serverBasePath, in order to be consistent with the server basePath service.

Having access to serverBasePath is certainly a niche requirement. A vast majority of applications won't need this, and should instead use the space-aware base path

[pgayvallet]

You are right.

Adding that to the injected metadata should be trivial.

(notes for later:)

Server

kibana/src/core/server/rendering/rendering_service.tsx

Line 62 in 7977f0e

const basePath = http.basePath.get(request);

kibana/src/core/server/http/base_path_service.ts

Lines 46 to 49 in 01ac90f

	public get = (request: KibanaRequest | LegacyRequest) => {
	const requestScopePath = this.basePathCache.get(ensureRawRequest(request)) || '';
	return `${this.serverBasePath}${requestScopePath}`;
	};

Client

kibana/src/core/public/injected_metadata/injected_metadata_service.ts

Lines 112 to 116 in 071db79

	public setup(): InjectedMetadataSetup {
	return {
	getBasePath: () => {
	return this.state.basePath;
	},

kibana/src/core/public/http/http_service.ts

Line 42 in af37d5f

const basePath = new BasePath(injectedMetadata.getBasePath());

kibana/src/core/public/http/base_path.ts

Lines 40 to 45 in 4980387

	export class BasePath {
	constructor(private readonly basePath: string = '') {}
	public get = () => {
	return this.basePath;
	};