refactor(falco_metrics): put content type in documented constant

This will keep things clearer and also allow for easir update in
the future.

Signed-off-by: Samuel Gaist <samuel.gaist@idiap.ch>

userspace/falco/falco_metrics.cpp

@@ -27,6 +27,15 @@ limitations under the License.
 	and falco libs into a string to be return by the metrics endpoint.
 */
 
+/*!
+	\brief content_type to be returned by the webserver's metrics endpoint.
+
+	Currently it is the default Prometheus exposition format
+
+	https://prometheus.io/docs/instrumenting/exposition_formats/#text-based-format
+*/
+const std::string falco_metrics::content_type = "text/plain; version=0.0.4";
+
 /*!
 	\brief Constructor that takes a \c state object to build its internal state
 */

userspace/falco/falco_metrics.h

@@ -27,6 +27,8 @@ namespace falco::app {
 class falco_metrics
 {
 public:
+	static const std::string content_type;
+
 	falco_metrics(falco::app::state& state);
 	bool is_enabled() const { return m_metrics_enabled; };
 	std::string to_text() const;

userspace/falco/webserver.cpp

@@ -69,7 +69,7 @@ void falco_webserver::start(
     {
         m_server->Get("/metrics",
             [metrics](const httplib::Request &, httplib::Response &res) {
-                res.set_content(metrics.to_text(), "text/plain; version=0.0.4");
+                res.set_content(metrics.to_text(), falco_metrics::content_type);
             });
     }
     // run server in a separate thread