DataDog have adopted a subtle naming convention:

 - dd-agent refers to the v5 Python implementation
 - datadog-agent refers to the v6 golang implementation

DataDog was dropping PID files and and python pickle files in
/tmp. Move these to private directories as a precaution.

This is the new v6 version of datadog-agent.

The old v5 module is kept as dd-agent.

Refactors the datadog-agent (i.e. V6) module to let users configure
arbitrary checks, not just a limited set, without having to resort to
linking the files manually and updating the systemd unit.

Checks are now configured via a `services.datadog-agent.checks` option
which takes an attribute set in which the keys refer directly to
Datadog check names, and the values are attribute sets representing
Datadog's configuration structure.

With this mechanism users can configure arbitrary integrations, for
example for the `ntp`-check, simply by saying:

    services.datadog-agent.checks.ntp = {
      init_config = null;
      # ... other check configuration options as per Datadog
      # documentation
    };

The previous check-specific configuration options for non-default
checks have been removed. Disk & network check configuration options
have been kept rather than making them a `default`-value of the
`checks`-option because they will be overridden by user-configurations
in that case.

Relates to #40399.

Refactors the process used to build the Datadog core integrations to
be more easily extensible with integrations other than the ones built
and installed by default.

Documentation has been added in relevant parts of the module to
describe how the process works.

As a high-level overview:

The `datadog-integrations-core` attribute in the top-level package set
now accepts an extra parameter.

This parameter is an attribute set where each key is the name of a
Datadog integration as it appears in Datadog's integrations-core
repository[1], and the value is a function that receives the Python
package set and returns the required dependencies of this integration.

For example:

    datadog-integrations-core {
      ntp = (ps: [ ps.ntplib ]);
    };

This would build the default integrations and, additionally, the `ntp`
integration.

To support passing the modified Python environment to the
datadog-agent itself, the `python` key has been moved inside of the
derivation which means that it will be made overridable.

This relates to #40399.

[1]: https://github.com/DataDog/integrations-core

Introduces an option `services.datadog-agent.extraIntegrations` that
can be set to include additional Datadog agent integrations from the
integrations-core repository.

Documentation and an example is provided with the change.

Relates to #40399

DataDog Agent v6