Prometheus reports 200 but Response is actually 500

Question

new to Prometheus here.

I added it to my Core2.1 project and Startup.cs I am doing:

    public static IApplicationBuilder UseCustomMetricsMiddleware(this IApplicationBuilder app)
    {
        app.UseHttpMetrics();

        var histogram = //...custom histogram
        var counter = //...custom counter 

        app.UseMiddleware<HttpRequestDurationMiddleware>(histogram);
        app.UseMiddleware<HttpRequestCountMiddleware>(counter);

        return app;
    }

This works and I get the histogram and the counter printing out. However when I do a POST that returns 500 in postman and in the logs:

HTTP POST /endpoint responded with 500 in 7506.6286 ms

Prometheus will print 200:

# TYPE http_request_counter counter
http_request_counter{code="200",method="POST"} 1

This is obviously wrong. The logs and Postman clearly show a 500 response, so I am trying to understand why Prometheus doesn't record the status correctly.

Any help is appreciated.

Thanks