Telegraf: Consider setting HOSTNAME using kubernetes downward api

### Expected behaviour

Telegraf uses the pod name as hostname by using the [Kubernetes Downward API](https://kubernetes.io/docs/concepts/workloads/pods/downward-api/). This can be latter used as an unique and human understandable indentifier on multiple plugins, like client id's in Kafka producers and consumers.

### Current behaviour

Hostname is manually set by using the `env` key on Telegraf values. By default, the env variable is set to "telegraf-polling-service", which is shared among all replicas.

Start by tracing the Helm chart's pod environment configuration and rendered Kubernetes manifests, focusing on how the HOSTNAME value is currently set. Verify that the rendered deployment uses the Kubernetes Downward API to expose each pod name and that replicas receive distinct, human-readable hostnames.

Written by the indexing model from the issue text.