Attach 'on subscribe' callback to observable

My angular 4 application has a rest service that is being injected into various components and returns observables. In one specific component I'd like to have a count of pending requests. I currently do this by incrementing a counter when a request is being made and decrementing it when the request has completed (successfully or not). Somewhat like this:

export class MyComponent {
    nRunningRequests = 0;

    constructor(private restService: RestService) {
    }

    loadData(type: string): Observable<any> {
        // this line is wrongly executed too soon, since the request
        // is not actually done yet
        this.nRunningRequests++;
        return this.restService.fetchData(type)
            .finally(() => this.nRunningOperations--);
    }

    loadNames(): Observable<any> {
        this.loadData('names');
        // oops, didn't subscribe, so no request is actually being done
        // but nRunningRequests is now wrong
    }
}

The problem I have is that if I don't actually subscribe() to the observable, no request is being triggered and therefore the counter shouldn't have been incremented. Is there a way to attach a callback to be called at subscription time? Something along the lines of:

    loadData(type: string): Observable<any> {
        return this.restService.fetchData(type)
            .initially(() => this.nRunningRequests++)
            .finally(() => this.nRunningRequests--);
    }

I could also move the counting logic into the rest service, but that doesn't make sense, because I only want to count the requests from this one component.