Add datadog instrumentation

[kalashnikovisme]

What is the purpose of this pull request?

AnyCable has provided StatsD in an open-source version since 1.3. Here are docs about Datadog

What changes did you make?

• Fixed typo in StatsD section
• Copied explanation of mem_sys_bytes metric to the main docs
• Added the list of metrics that AnyCable sends by default via StatsD
• Added the short description and the link to Datadog + Heroku page

Checklist

• [x] I've updated the documentation

[kalashnikovisme]

I have decided to close this and create a new one for a cleaner history. https://github.com/anycable/anycable-go/pull/167