intercepter-2.ts

import { CallHandler, ExecutionContext, Injectable, NestInterceptor } from '@nestjs/common';
import { Observable } from 'rxjs';
import { flatMap } from 'rxjs/operators';
import { fromFunctionCall } from '../common/utils';

@Injectable()
export class LoginInterceptor implements NestInterceptor {
  intercept(context: ExecutionContext, next: CallHandler): Observable<any> {
    const req = context.switchToHttp().getRequest();
    return fromFunctionCall(req.logIn, req.user).pipe(
      flatMap(() => next.handle())
    );
  }

utils.ts

import { Observable } from "rxjs";

type Fn<A extends any[], R> = (...args: A) => R;
type Cb<E extends Error, R> = (err?: E | null, result?: R) => void;

export const fromFunctionCall = <A extends any[], E extends Error, R>(
    fn: Fn<[...A, Cb<E, R>], void>, 
    ...args: A
): Observable<R> => new Observable(
    observer => fn(...args, (err?: E | null, result?: R) => {
        if (err) {
            observer.error(err);
            return;
        }
        observer.next(result);
        observer.complete();
    })
);