docs: pipeline: filters: kubernetes: document kube_namespace_file option

Add the kube_namespace_file option to the configuration table. The
filter reads the pod namespace from this configurable path instead of
a hard-coded location.

Note this update due to code change without matching docs PR.

Signed-off-by: Eric D. Schabell <eric@schabell.org>

Author: eschabell

pipeline/filters/kubernetes.md

@@ -55,6 +55,7 @@ The plugin supports the following configuration parameters:
 | `kube_meta_cache_ttl` | Configurable time-to-live for Kubernetes cached pod metadata. By default, it's set to `0` which means `TTL` for cache entries is disabled and cache entries are evicted at random when capacity is reached. To enable this option, set the number to a time interval. For example, set the value to `60` or `60s` and cache entries which have been created more than 60 seconds ago will be evicted. | `0` |
 | `kube_meta_namespace_cache_ttl` | Configurable time-to-live for Kubernetes cached namespace metadata. If set to `0`, entries are evicted at random when capacity is reached. | `900` (seconds) |
 | `kube_meta_preload_cache_dir` | If set, Kubernetes metadata can be cached or pre-loaded from files in JSON format in this directory, named `namespace-pod.meta`. | _none_ |
+| `kube_namespace_file` | Path to the file that contains the pod's namespace. | `/var/run/secrets/kubernetes.io/serviceaccount/namespace` |
 | `kube_tag_prefix` | When the source records come from the `tail` input plugin, this option specifies the prefix used in `tail` configuration. | `kube.var.log.containers.` |
 | `kube_token_command` | Command to get Kubernetes authorization token. Defaults to `NULL` uses the token file to get the token. To manually choose a command to get it, set the command here. For example, run `aws-iam-authenticator -i your-cluster-name token --token-only` to set token. This option is currently Linux-only. | `NULL` |
 | `kube_token_file` | Token file | `/var/run/secrets/kubernetes.io/serviceaccount/token` |