Add forward metrics

[therc]

What does this PR do?

Add CoreDNS metrics for the forward plugin

Motivation

The proxy plugin is deprecated and has been removed in recent versions, with users expected to replace it with the forward plugin.

Additional Notes

I couldn't grab data for all tests: healtcheck failures are missing.

Review checklist (to be filled by reviewers)

• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached
• Feature or bugfix must have tests
• Git history must be clean
• If PR adds a configuration option, it must be added to the configuration file.

[therc]

The Docker test fails because it runs an ancient version of CoreDNS, 1.2.0. Should there be two Docker tests?

[therc]

I got it to work by changing the configuration to use both plugins and triggering two DNS lookups.

[hithwen]

You can try including another coredns version to test in the tox file:

    py{27,37}-{1.2.0,1.6.4}

and then adapting the test for different versions.
You can find examples in other integrations that are version based like postgres

[therc]

It doesn't look like we need to test two versions, the forward plugin already existed in 1.2.0.

[therve]

Looking good, some minor comments.

[therve]

Can you pass "HOST" instead of "127.0.0.1" ?

[therc]

Done

[therve]

Is that not a useful metric to surface?

[therc]

Added

[therc]

Can this be merged now? Thanks.

[FlorianVeaux]

Merging and releasing the integration, thanks!
PS: The PR title is used in coredns changelog as is, I removed 'coredns' from it as we don't need to specify the integration name here.

[FlorianVeaux]

This has been released and will be automatically included in Agent 6.16, until then you can install it manually with the command datadog-agent integration install datadog-coredns==1.3.0