Missing Content-Type header for metrics

[mgcrea]

Description

Not really a bug but the http response of the prometheus endpoint lacks a Content-Type header (should be text/plain). Unfortunately influxdb2 fails to properly scrap the endpoint without that.

Version

sigp/lighthouse:v1.0.2

Present Behaviour

http -h http://172.17.0.1:5054/metrics
HTTP/1.1 200 OK
content-length: 40138
date: Sun, 29 Nov 2020 19:45:14 GMT
server: Lighthouse/v1.0.2-f7183098e/x86_64-linux

Expected Behaviour

http -h http://172.17.0.1:5054/metrics
HTTP/1.1 200 OK
content-length: 40138
content-type: text/plain; charset=utf-8
date: Sun, 29 Nov 2020 19:45:14 GMT
server: Lighthouse/v1.0.2-f7183098e/x86_64-linux

Exemple Geth output:

http -h http://172.17.0.1:6060/debug/metrics/prometheus
HTTP/1.1 200 OK
Content-Length: 46840
Content-Type: text/plain
Date: Sun, 29 Nov 2020 19:44:46 GMT

[AgeManning]

Resolved in #2019