Wait for localstorage inside http observable?

[rapropos]

interface Instance {
  // don't know what goes here, but don't want to perpetuate `any` abuse
}

getInstances(): Observable<Instance[]> {
  return from(this.localStorage.getEtagInstances()).pipe(
    switchMap(etag => {
      let headers = new HttpHeaders().append("accept-language", localLanguage);
      if (etag) { // i avoid direct comparison against null as much as possible
        headers = headers.append("If-None-Match", etag);
      }
      return this.http.get<Instance[]>(BACKEND_URL + "/api/ROUTE", 
        {headers, params, observe: "response"});
    }),
    tap(rsp => this.localStorage.setEtagInstances(rsp.headers.get("etag")))
  );
}