Adjust Content-Type header for Prometheus endpoint to include version

Prometheus v3 will, by default, be more strict about the content-types
returned from scrape endpoints. With the current value (just
`text/plain`), it would fail to scrape.

In this commit the value is changed from `text/plain` to `text/plain;
version=0.0.4`.

See also [1] and [2]
[1] https://prometheus.io/docs/instrumenting/exposition_formats/
[2] https://prometheus.io/docs/prometheus/3.0/migration/

(cherry picked from commit f572f31)

pdns/dnsdistdist/dnsdist-web.cc

@@ -913,7 +913,7 @@ static void handlePrometheus(const YaHTTP::Request& req, YaHTTP::Response& resp)
   output << "dnsdist_info{version=\"" << VERSION << "\"} " << "1" << "\n";
 
   resp.body = output.str();
-  resp.headers["Content-Type"] = "text/plain";
+  resp.headers["Content-Type"] = "text/plain; version=0.0.4";
   // clang-format on
 }
 #endif /* DISABLE_PROMETHEUS */

pdns/recursordist/docs/http-api/prometheus.rst

@@ -18,7 +18,7 @@ Prometheus Data Endpoint
     HTTP/1.1 200 OK
     Connection: close
     Content-Length: 19203
-    Content-Type: text/plain
+    Content-Type: text/plain; version=0.0.4
     Server: PowerDNS/0.0.16480.0.g876dd46192
 
     # HELP pdns_recursor_all_outqueries Number of outgoing UDP queries since starting

pdns/recursordist/ws-recursor.cc

@@ -564,7 +564,7 @@ static void prometheusMetrics(HttpRequest* /* req */, HttpResponse* resp)
          << "\n";
 
   resp->body = output.str();
-  resp->headers["Content-Type"] = "text/plain";
+  resp->headers["Content-Type"] = "text/plain; version=0.0.4";
   resp->status = 200;
 }
 

pdns/ws-auth.cc

@@ -2667,7 +2667,7 @@ static void prometheusMetrics(HttpRequest* /* req */, HttpResponse* resp)
          << "\n";
 
   resp->body = output.str();
-  resp->headers["Content-Type"] = "text/plain";
+  resp->headers["Content-Type"] = "text/plain; version=0.0.4";
   resp->status = 200;
 }