Improve container checker for gnmi/telemetry container (#18529)

### Why I did it
We have used gnmi container to replace telemetry container, and telemetry is still enabled after upgrade.
container_checker script reads from features table and check if the container is running, telemetry is enabled but there's no telemetry container.
It's difficult to disable telemetry in feature table for warm reboot and cold reboot, we need to check docker image in db migrator and minigraph.py.

### How I did it
I modify container_checker script:
If there's docker-sonic-telemetry image, check telemetry container.
If there's no docker-sonic-telemetry image, check gnmi container instead.
If there's no docker-sonic-telemetry image and docker-sonic-gnmi image, do not check telemetry.

#### How to verify it
Run end to end test with cold-reboot and warm-reboot.

files/image_config/monit/container_checker

@@ -25,6 +25,19 @@ from swsscommon import swsscommon
 EVENTS_PUBLISHER_SOURCE = "sonic-events-host"
 EVENTS_PUBLISHER_TAG = "event-down-ctr"
 
+def check_docker_image(image_name):
+    """
+    @summary: This function will check if docker image exists.
+    @return:  True if the image exists, otherwise False.
+    """
+    try:
+        DOCKER_CLIENT = docker.DockerClient(base_url='unix://var/run/docker.sock')
+        DOCKER_CLIENT.images.get(image_name)
+        return True
+    except (docker.errors.ImageNotFound, docker.errors.APIError) as err:
+        print("Failed to get image '{}'. Error: '{}'".format(image_name, err))
+        return False
+
 def get_expected_running_containers():
     """
     @summary: This function will get the expected running & always-enabled containers by following the rule:
@@ -55,7 +68,24 @@ def get_expected_running_containers():
     # it will be removed from exception list.
     run_all_instance_list = ['database', 'bgp']
 
+    container_list = []
     for container_name in feature_table.keys():
+        # slim image does not have telemetry container and corresponding docker image
+        if container_name == "telemetry":
+            ret = check_docker_image("docker-sonic-telemetry")
+            if not ret:
+                # If telemetry container image is not present, check gnmi container image
+                # If gnmi container image is not present, ignore telemetry container check
+                # if gnmi container image is present, check gnmi container instead of telemetry
+                ret = check_docker_image("docker-sonic-gnmi")
+                if not ret:
+                    print("Ignoring telemetry container check on image which has no corresponding docker image")
+                else:
+                    container_list.append("gnmi")
+                continue
+        container_list.append(container_name)
+
+    for container_name in container_list:
         if feature_table[container_name]["state"] not in ["disabled", "always_disabled"]:
             if multi_asic.is_multi_asic():
                 if feature_table[container_name].get("has_global_scope", "True") == "True":