fix(plugins/ecs): ECS v3 endpoint environment var is documented with …

…debug info

Fix ECS plugin to inform user when v3 endpoint is unavailable (no found
environment variable)

issue #CIRC-8361

* Tags: ecs input plugin v3 env

plugins/inputs/ecs/ecs.go

@@ -3,6 +3,7 @@ package ecs
 import (
 	"context"
 	"fmt"
+	"log"
 	"os"
 	"strings"
 	"time"
@@ -163,6 +164,8 @@ func resolveEndpoint(ecs *Ecs) {
 		return
 	}
 
+	log.Print("I! ECS_CONTAINER_METADATA_URI Environment var is unset, using v2 ecs endpoint")
+
 	// Use v2 endpoint if nothing else is available.
 	ecs.EndpointURL = v2Endpoint
 	ecs.metadataVersion = 2