Update EC2 SD docs

FIxing docs to match available labels in EC2 sd.

[https://github.com/prometheus/prometheus/blame/master/retrieval/discovery/ec2.go](https://github.com/prometheus/prometheus/blame/master/retrieval/discovery/ec2.go)

content/docs/operating/configuration.md

@@ -476,10 +476,16 @@ the public IP address with relabeling.
 
 The following meta labels are available on targets during relabeling:
 
+* `__meta_ec2_availability_zone`: the availability zone in which the instance is running
 * `__meta_ec2_instance_id`: the EC2 instance ID
-* `__meta_ec2_public_ip`: the public IP address of the instance
 * `__meta_ec2_private_ip`: the private IP address of the instance, if present
+* `__meta_ec2_public_dns_name`: the public DNS name of the instance, if available
+* `__meta_ec2_public_ip`: the public IP address of the instance, if available
+* `__meta_ec2_subnet_id`: comma separated list of subnets IDs in which the instance is running, if available
 * `__meta_ec2_tag_<tagkey>`: each tag value of the instance
+* `__meta_ec2_vpc_id`: the ID of the VPC in which the instance is running, if available
+
+
 
 See below for the configuration options for EC2 discovery: