add validity time parameter

Readme.md

@@ -24,6 +24,7 @@ The payload needs to be an object containing only the following keys:
 - `name` (string): the name of the metric
 - `value` (number): the value of the metric
 - `labels` (object, optional): an object containing the labels for the metric (key-value pairs of strings)
+- `valid_seconds` (number, optional, defaults to 60): the number of seconds the metric is valid. After this time, the metric will be removed from the export. (which will results in a gap in prometheus)
 
 According to prometheus conventions, the metric name and the label names need to be a lower or upper case letter or a underscore followed by all case leters, numbers or underscores.
 

mqtt2prom/run

@@ -173,8 +173,25 @@ function precheck(Message $message): bool
             return false;
         }
     }
+
+    if(!isset($payload["valid_seconds"])){
+        $payload["valid_seconds"] = 60;
+    }
+    if(!is_numeric($payload["valid_seconds"])){
+        error("valid_seconds must be a number");
+        return false;
+    }
+    if($payload["valid_seconds"] < 1){
+        error("valid_seconds must be at least 1");
+        return false;
+    }
+    if($payload["valid_seconds"] >60*60*48){
+        error("valid_seconds must be at most 48 hours");
+        return false;
+    }
+
     foreach (array_keys($payload) as $key) {
-        if (!in_array($key, ["name", "value", "labels"])) {
+        if (!in_array($key, ["name", "value", "labels","valid_until"])) {
             error("Unknown key: $key");
             return false;
         }
@@ -307,7 +324,7 @@ function filter(): void
 {
     global $data, $stats;
     $data = array_filter($data, function ($entry) {
-        return $entry["timestamp"] > time() - 60;
+        return $entry["timestamp"] > time() - $entry["valid_seconds"];
     });
     // remove stats older than 24h
     foreach ($stats as $key => $values) {