Add support for DD_ENHANCED_METRICS variable in forwarder template

[LorisFriedel]

What does this PR do?

Update the datadog forwarder template to support the passing of DD_ENHANCED_METRICS boolean.

Motivation

The goal is to be able to enable enhanced metrics for the lambda itself, for better observability of the lambda.

Types of changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog
• This PR passes the integration tests (ask a Datadog member to run the tests)
• This PR passes the unit tests
• This PR passes the installation tests (ask a Datadog member to run the tests)

[RaphaelAllier]

cc @ge0Aja Can you check #687 that the discussions there don't contradict the changes in this PR ? There is a serverless impact to doing this

[ge0Aja]

cc @ge0Aja Can you check #687 that the discussions there don't contradict the changes in this PR ? There is a serverless impact to doing this

Yep, we should be fine, DD_ENHANCED_METRICS=false by default.

[ge0Aja]

lgtm