Missing `Content-Type` in `HttpResponse`?

[rkleinem]

Hi,

the struct PrometheusMetricsHandler implements actix_web::Handler.
It constructs an HttpResponse here:

actix-web-opentelemetry/src/middleware/metrics.rs

Lines 348 to 353 in a883322

	fn call(&self, _req: actix_web::HttpRequest) -> Self::Future {
	Box::pin(future::ok(actix_web::HttpResponse::with_body(
	StatusCode::OK,
	self.metrics(),
	)))
	}

I think this header should have a Content-Type field as explained here: https://prometheus.io/docs/instrumenting/exposition_formats/.
(I.e. content-type: text/plain)
It seems that this became a real issue with Prometheus 3: prometheus/prometheus#15485.

I don't have a full overview of the ecosystem yet, so I hope this is the right place.
If you want I could make a PR for this.

Cheers