ISSUE-2161: When httpServerEnabled=true, Prometheus reports doesn't set content-type

[sijie]

Original Issue: apache#2161

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BUG REPORT

Describe the bug

When running with httpServerEnabled set to true, /metrics endpoint stops sending a content-type header (this uses the MetricsService, see code here https://github.com/apache/bookkeeper/blob/28dee8464764b0edceeb31ce24424f0947b5789b/bookkeeper-server/src/main/java/org/apache/bookkeeper/server/http/service/MetricsService.java).
With the httpServerEnabled=false the PrometheusServlet is used, and it does set a content-type header (https://github.com/apache/bookkeeper/blob/28dee8464764b0edceeb31ce24424f0947b5789b/bookkeeper-stats-providers/prometheus-metrics-provider/src/main/java/org/apache/bookkeeper/stats/prometheus/PrometheusServlet.java#L44)

The content-type header is expected by some prometheus tools to differentiate between plaintext and protobuf formatted metrics and no content-type is considered an error.

To Reproduce

Steps to reproduce the behavior:

1. Set httpServerEnabled=false and enable prometheus reporter
2. run curl localhost:8000/metrics and notice that a content-type header is set
3. Set httpServerEnabled=true and enable prometheus reporter
4. run curl localhost:8000/metrics -v notice that there is no content-type header set

Expected behavior

I expect a content-type header

It seems like the statsProvider interface should have a method for allowing to set headers on the response. Default to a given content type might cause problems with other providers.