feat: add ability to delay startup (#631)

* Feat: add ability to delay startup

* Enable ability to set delay seconds before start up gatus

* Update README.md

* Delete .examples/delay-startup/Makefile

---------

Co-authored-by: TwiN <twin@linux.com>

README.md

@@ -101,6 +101,7 @@ Have any feedback or questions? [Create a discussion](https://github.com/TwiN/ga
   - [Endpoint groups](#endpoint-groups)
   - [Exposing Gatus on a custom path](#exposing-gatus-on-a-custom-path)
   - [Exposing Gatus on a custom port](#exposing-gatus-on-a-custom-port)
+  - [Configuring a startup delay](#configuring-a-startup-delay)
   - [Keeping your configuration small](#keeping-your-configuration-small)
   - [Badges](#badges)
     - [Uptime](#uptime)
@@ -1864,6 +1865,10 @@ web:
 ```
 
 
+### Configuring a startup delay
+If, for any reason, you need Gatus to wait for a given amount of time before monitoring the endpoints on application start, you can use the `GATUS_DELAY_START_SECONDS` environment variable to make Gatus sleep on startup.
+
+
 ### Keeping your configuration small
 While not specific to Gatus, you can leverage YAML anchors to create a default configuration.
 If you have a large configuration file, this should help you keep things clean.

main.go

@@ -4,6 +4,7 @@ import (
 	"log"
 	"os"
 	"os/signal"
+	"strconv"
 	"syscall"
 	"time"
 
@@ -14,6 +15,10 @@ import (
 )
 
 func main() {
+	if delayInSeconds, _ := strconv.Atoi(os.Getenv("GATUS_DELAY_START_SECONDS")); delayInSeconds > 0 {
+		log.Printf("Delaying start by %d seconds", delayInSeconds)
+		time.Sleep(time.Duration(delayInSeconds) * time.Second)
+	}
 	cfg, err := loadConfiguration()
 	if err != nil {
 		panic(err)