docs/configuration: clarify OpenStack metadata labels (#14149)

On several occasions, users assumed that the
`__meta_openstack_tag_<key>` labels were about tags [1] instead of
metadata [2]. While we can't really change the Prometheus label name, we
can at least clarify in the documentation what's the information carried
in the label.

[1] https://specs.openstack.org/openstack/api-wg/guidelines/tags.html
[2] https://docs.openstack.org/api-ref/compute/#server-metadata-servers-metadata

Signed-off-by: Simon Pasquier <spasquie@redhat.com>

docs/configuration/configuration.md

@@ -1349,7 +1349,7 @@ interface.
 The following meta labels are available on targets during [relabeling](#relabel_config):
 
 * `__meta_openstack_address_pool`: the pool of the private IP.
-* `__meta_openstack_instance_flavor`: the flavor of the OpenStack instance.
+* `__meta_openstack_instance_flavor`: the flavor ID of the OpenStack instance.
 * `__meta_openstack_instance_id`: the OpenStack instance ID.
 * `__meta_openstack_instance_image`: the ID of the image the OpenStack instance is using.
 * `__meta_openstack_instance_name`: the OpenStack instance name.
@@ -1357,7 +1357,7 @@ The following meta labels are available on targets during [relabeling](#relabel_
 * `__meta_openstack_private_ip`: the private IP of the OpenStack instance.
 * `__meta_openstack_project_id`: the project (tenant) owning this instance.
 * `__meta_openstack_public_ip`: the public IP of the OpenStack instance.
-* `__meta_openstack_tag_<tagkey>`: each tag value of the instance.
+* `__meta_openstack_tag_<key>`: each metadata item of the instance, with any unsupported characters converted to an underscore.
 * `__meta_openstack_user_id`: the user account owning the tenant.
 
 See below for the configuration options for OpenStack discovery: