Give option to not append `name_prefix` if the metric name already begins with said prefix

## Feature Request

### Proposal:

That Telegraf adds an option to inputs to avoid prefixing `name_prefix` if the metrics already begin with the prefix.

### Current behavior:

Telegraf does not have an option to avoid prefixing `name_prefix` when metrics already begin with the prefix.

### Desired behavior:

Telegraf has an option to avoid prefixing `name_prefix` when metrics already begin with the prefix.

### Use case:

Generally, I would appreciate this option as I find end-users misusing `name_prefix` for applications that already emit metrics with that prefix, e.g. `name_prefix = prometheus` for a Prometheus server producing `prometheus_prometheus.*`.

More importantly though, an end-user was observed misconfiguring their `telegraf.conf` such that the Prometheus input was scraping the Prometheus output client. Since they were using `name_prefix` on their input, which was in essence their output, it was essentially continually/recursively prefixing `name_prefix`. While other safeguards like sample limits can be and are in place, and while this is like a hella corner case, I would like to be able to protect against it in future. I happened to catch it on a spot check.

I'm happy to code this up if the proposal is accepted 👍

No files, tests, or entry points are named. Start by locating Telegraf's handling of input name_prefix and metric names, then determine where the option belongs and how existing configuration is tested. Done means an input can avoid adding name_prefix when the metric name already begins with it, including the described recursive-prefix case.

Written by the indexing model from the issue text.