Add support for gzip compression to amqp input and output

[danielnelson]

closes #3557

Required for all PRs:

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

[danielnelson]

• telegraf-1.11.0~97f47699-0.x86_64.rpm
• telegraf_1.11.0~97f47699-0_amd64.deb
• telegraf-1.11.0~97f47699_windows_amd64.zip

[Esity]

We should add in the compression message property so we know in RMQ if it is compressed. This should be set based on if compression is enabled or not.

Content encoding, e.g. "gzip". Used by applications, not core RabbitMQ
While we are at it, we should also add one more

Content type, e.g. "application/json". Used by applications, not core RabbitMQ
Grabbed from this page https://www.rabbitmq.com/consumers.html

[Esity]

Also tested with and without gzip compression
I added 10k messages to RMQ without 10k metric points per message for a total of 10,000,000 metrics.

Without compression the size on disk was over 4gb. With compression turned on it was 7kB. I would consider that a success @danielnelson

It might be worth testing to see what the additional overhead against Telegraf is for enabling compression but I think it is worth it

[danielnelson]

I added the Content-Encoding property, but held off on the Content-Type. I think we should first make this part of the serializer interface separately so that we can pick the right value in a nice way. Another idea that this brings up is doing automatic decoding on the consumer side based on the property, I think it could be nice but I'll have to leave that for later.

• telegraf-1.11.0~543178e3-0.x86_64.rpm
• telegraf_1.11.0~543178e3-0_amd64.deb
• telegraf-1.11.0~543178e3_windows_amd64.zip