Add content-type header to metrics server response (#3970)

This fixes issues with certain metrics scrapers, which might error if the content-type is not correctly set.

## Issue Addressed

Fixes https://github.com/sigp/lighthouse/issues/3437

## Proposed Changes

Simply set header: `Content-Type: text/plain` on metrics server response. Seems like the errored branch does this correctly already.

## Additional Info

This is needed also to enable influx-db metric scraping which work very nicely with Geth.

validator_client/src/http_metrics/mod.rs

@@ -121,7 +121,13 @@ pub fn serve<T: EthSpec>(
         .and_then(|ctx: Arc<Context<T>>| async move {
             Ok::<_, warp::Rejection>(
                 metrics::gather_prometheus_metrics(&ctx)
-                    .map(|body| Response::builder().status(200).body(body).unwrap())
+                    .map(|body| {
+                        Response::builder()
+                            .status(200)
+                            .header("Content-Type", "text/plain")
+                            .body(body)
+                            .unwrap()
+                    })
                     .unwrap_or_else(|e| {
                         Response::builder()
                             .status(500)