do-agent on FreeBSD exports only CPU, load, disk usage and bandwidth metric

## Describe the problem
The (with fix #236) build do-agent exports only CPU, load, disk usage and bandwidth metric, at least thats what is shown in the Graphs on the webinterface.

## Steps to reproduce
create a FreeBSD droplet, install the do-agent (steps to build it are in #236), run the agent e.g:
`do-agent-freebsd-amd64 --debug --collector.cpu --collector.devstat --collector.zfs --collector.meminfo`
check the graphs in the webinterface
## Expected behavior
All relevant metrics are shown in the webinterface

## System Information
Distribution and version:

### do-agent information:
FreeBSD 12.2-RELEASE-p2
```
./do-agent-freebsd-amd64 -v

do-agent (DigitalOcean Agent)

Version: 3.8.0
Revision: 3b03160
Build Date: So. 27 Dez. 2020 15:05:38 UTC
Go Version: go1.15.6
Website: https://github.com/digitalocean/do-agent

Copyright (c) 2020 DigitalOcean, Inc. All rights reserved.

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