do-agent --collector.<name> flags are confusing

## Describe the problem

While experimenting with do-agent, I wanted to see if do-agent could support additional metrics for monitoring, graphing and alerting on DigitalOcean. I ran `do-agent --help` and saw options for enabling collectors that look familiar from the prometheus node_exporter to me. I tried to enable a collector using command-line flags, and received an error that I had to check the source code to understand.

## Steps to reproduce
When I run `do-agent --help`, I see many `--collector.` options, e.g.:

```
...
--collector.arp Enable the arp collector (default: enabled).
--collector.bcache Enable the bcache collector (default: enabled).
...
```
However, if I try to use these flags, e.g.:
```console
# do-agent --collector.arp
do-agent: error: flag 'collector.arp' cannot be repeated, try --help
```
This didn't make sense to me as I had only specified `--collector.arp` once.

Also, the help text for `--collector.arp` says `(default: enabled)`, however the collector is disabled for my platform https://github.com/digitalocean/do-agent/blob/master/cmd/do-agent/config_linux.go because disables arp.

I get the same results with the `--no-collector.arp` flag too:

```console
# do-agent --no-collector.arp
do-agent: error: flag 'collector.arp' cannot be repeated, try --help
```
I think that the `--no-collector.` options should be kept for the enabled collectors, e.g.: ` --no-collector.cpu`.

## Expected behavior

* `--collector.arp` would enable the arp collector
* `--collector.arp` help text would show the default for the current platform

OR

* The `--collector.arp` flag would not be valid for do-agent
* `--collector.arp` help text including the default would not be shown

## System Information
Distribution and version:

Debian GNU/Linux 10 (buster)

### do-agent information:

```

do-agent (DigitalOcean Agent)

Version: 3.9.4
Revision: 766c949
Build Date: Fri Mar 12 20:37:53 UTC 2021
Go Version: go1.16.2
Website: https://github.com/digitalocean/do-agent

Copyright (c) 2021 DigitalOcean, Inc. All rights reserved.

This work is licensed under the terms of the Apache 2.0 license.
For a copy, see .

```