Avoid stale connections by pinging on acquire

The exporter was returning cached MongoDB connections without validating their health. This meant that if a connection had become unhealthy,
subsequent operations would fail after a delay (serverSelectionTimeout etc), causing the entire request to be slower than it should be.

This commit adds  `Ping` check before returning a cached client.

Author: impimp

exporter/exporter.go

@@ -273,8 +273,13 @@ func (e *Exporter) getClient(ctx context.Context) (*mongo.Client, error) {
 		e.clientMu.Lock()
 		defer e.clientMu.Unlock()
 
-		// If client is already initialized, return it.
+		// If client is already initialized, and Ping is succesful -- return it.
 		if e.client != nil {
+			err := e.client.Ping(ctx, nil)
+			if err != nil {
+				return nil, err
+			}
+
 			return e.client, nil
 		}