add metric: cert_expire_timestamp

inputs/http_response/http_response.go

@@ -309,6 +309,11 @@ func (ins *Instance) httpGather(target string) (map[string]string, map[string]in
 		fields["result_code"] = Success
 	}
 
+	// check tls cert
+	if strings.HasPrefix(target, "https://") && resp.TLS != nil {
+		fields["cert_expire_timestamp"] = getEarliestCertExpiry(resp.TLS).Unix()
+	}
+
 	defer resp.Body.Close()
 
 	// metric: response_code

inputs/http_response/tls.go

@@ -0,0 +1,16 @@
+package http_response
+
+import (
+	"crypto/tls"
+	"time"
+)
+
+func getEarliestCertExpiry(state *tls.ConnectionState) time.Time {
+	earliest := time.Time{}
+	for _, cert := range state.PeerCertificates {
+		if (earliest.IsZero() || cert.NotAfter.Before(earliest)) && !cert.NotAfter.IsZero() {
+			earliest = cert.NotAfter
+		}
+	}
+	return earliest
+}