Handle labels in kube_inventory input like the kubernetes input does

## Feature Request

The Kubernetes input plugin can turn pod labels into tags on the `kubernetes_pod_container` measurement. For example, if you set `label_include = ["app.kubernetes.io/name"]`, then all your measurements will get a tag with the value of that field.

The Kubernetes Inventory plugin _also_ produces a measurement named `kubernetes_pod_container`, but this feature isn't available for that plugin. This makes it harder than necessary to aggregate across these two metrics sources (for example, to compare the `kubernetes_pod_container.memory_usage_bytes` field from the `kubernetes` plugin with the `kubernetes_pod_container.resource_requests_memory_bytes` field from the `kube_inventory` plugin.

### Proposal:

Copy the `label_include` and `label_exclude` functionality from the `kubernetes` plugin to the `kube_inventory` plugin.

...or just merge them into a single plugin, if at all possible; having them as separate plugins (that can't even really be deployed together) is kind of a PITA.

### Current behavior:
`kube_inventory` plugin doesn't allow you to turn labels into tags.

### Desired behavior:
`kube_inventory` plugin allows you to turn labels into tags, just like the `kubernetes` plugin does.

### Use case:
I want to build a dashboard in InfluxDB that displays current resource usage, resource request, and resource limit, all on the same graph. However, I want to be able to use a filter to limit the displayed metrics to pods of a given label value. I can already do this for the metrics generated by the `kubernetes` plugin (aka the resource usage), but I can't do it for metrics from the `kube_inventory` plugin (the requests and limits).

Start by locating the kubernetes and kube_inventory input plugin implementations and compare how label_include and label_exclude are handled in the kubernetes plugin. Trace the related plugin tests or test fixtures before making the behavior consistent. Done means kube_inventory can expose selected pod labels as tags on kubernetes_pod_container while excluding configured labels, with coverage for the new behavior.

Written by the indexing model from the issue text.