Add support for glob patterns in net input plugin

[bobmshannon]

An approach for fixing #1073.

Adds support for specifying golang glob-style patterns.

Example

[[inputs.net]]
   interfaces = ["en*", "utun[0-1]", "lo0"]

$ ./telegraf --input-filter net --config telegraf.conf --test
* Plugin: inputs.net, Collection 1
> net,interface=lo0,host=mac bytes_sent=103240327i,packets_sent=227866i,drop_in=0i,drop_out=0i,bytes_recv=103240327i,packets_recv=227866i,err_in=0i,err_out=0i 1503126134000000000
> net,interface=en0,host=macbook packets_sent=13568823i,packets_recv=23715254i,err_in=0i,err_out=0i,drop_out=3193i,bytes_sent=2411560838i,bytes_recv=30527805468i,drop_in=0i 1503126134000000000
> net,interface=en1,host=macbook bytes_sent=0i,drop_out=0i,bytes_recv=0i,packets_sent=0i,packets_recv=0i,err_in=0i,err_out=0i,drop_in=0i 1503126134000000000
> net,interface=en2,host=macbook bytes_recv=0i,packets_sent=0i,packets_recv=0i,err_in=0i,err_out=0i,drop_in=0i,bytes_sent=0i,drop_out=0i 1503126134000000000
> net,interface=utun0,host=macbook bytes_sent=268i,packets_sent=3i,err_in=0i,drop_in=0i,drop_out=0i,bytes_recv=0i,packets_recv=0i,err_out=0i 1503126134000000000
> net,interface=utun1,host=macbook bytes_sent=22156i,packets_recv=115i,drop_in=0i,drop_out=0i,bytes_recv=20275i,packets_sent=108i,err_in=0i,err_out=0i 1503126134000000000

Required for all PRs:

• Signed CLA.
• Associated README.md updated.
• Has appropriate unit tests.

[ljagiello]

@danielnelson any chance this PR will be merge before 1.5.0?

[danielnelson]

I think we can get this in, looking over the code I assume the code causing the performance issue is caused by calling net.InterfaceByName(io.Name), which is only called if the interfaces option is empty. So by adding the ability to use globs we bypass that code? If either of you are experiencing the performance issue can you verify my assumption?

[danielnelson]

We already have this (line 21)

[danielnelson]

Use filter.Filter here

[bobmshannon]

I think the performance problem can currently be mitigated by explicitly specifying a list of interfaces, it's just that support for glob patterns makes doing so less verbose and more adapting to different environments (e.g. eth* vs eth0,eth1,eth2,eth3,...)

[ljagiello]

@bobmshannon It would be super cool if you could add blacklist, I would love to blacklist all calico interfaces.

[bobmshannon]

@danielnelson I have addressed the PR comments, but it looks like the build may have flaked.

@ljagiello A blacklist sounds like an interesting idea. I'm not sure I can get that in this PR but it may be worth opening a separate issue for. @danielnelson, what do you think about such an option?

[danielnelson]

We can add the blacklist, usually we have a pair of options like interface_include interface_exclude. We could add this in a backwards compatible way by appending the interfaces option to interface_include.

[danielnelson]

I think the tests are failing because the branch is so far behind master. Failures are in unrelated code though so I don't mind merging.