res_prometheus.c: Set Content-Type header on /metrics response.

This should resolve the Prometheus error:

> Error scraping target: non-compliant scrape target
  sending blank Content-Type and no
  fallback_scrape_protocol specified for target.

Resolves: #1075

res/res_prometheus.c

@@ -603,6 +603,7 @@ static int http_callback(struct ast_tcptls_session_instance *ser,
 {
 	RAII_VAR(struct module_config *, mod_cfg, ao2_global_obj_ref(global_config), ao2_cleanup);
 	struct ast_str *response = NULL;
+	struct ast_str *content_type_header = NULL;
 	struct timeval start;
 	struct timeval end;
 
@@ -635,10 +636,13 @@ static int http_callback(struct ast_tcptls_session_instance *ser,
 	}
 
 	response = ast_str_create(512);
-	if (!response) {
+	content_type_header = ast_str_create(32);
+	if (!response || !content_type_header) {
 		goto err500;
 	}
 
+	ast_str_set(&content_type_header, 0, "Content-Type: text/plain\r\n");
+
 	start = ast_tvnow();
 
 	ast_mutex_lock(&scrape_lock);
@@ -659,7 +663,7 @@ static int http_callback(struct ast_tcptls_session_instance *ser,
 	}
 	ast_mutex_unlock(&scrape_lock);
 
-	ast_http_send(ser, method, 200, "OK", NULL, response, 0, 0);
+	ast_http_send(ser, method, 200, "OK", content_type_header, response, 0, 0);
 
 	return 0;
 
@@ -678,14 +682,17 @@ err401:
 		ast_http_send(ser, method, 401, "Unauthorized", auth_challenge_headers, NULL, 0, 1);
 	}
 	ast_free(response);
+	ast_free(content_type_header);
 	return 0;
 err503:
 	ast_http_send(ser, method, 503, "Service Unavailable", NULL, NULL, 0, 1);
 	ast_free(response);
+	ast_free(content_type_header);
 	return 0;
 err500:
 	ast_http_send(ser, method, 500, "Server Error", NULL, NULL, 0, 1);
 	ast_free(response);
+	ast_free(content_type_header);
 	return 0;
 }