`ENABLE_HEROKU_POSTGRES` (etc.) environment variables do not follow `DD_*` convention

[nertzy]

All of the environment variables that are used to configure the Datadog agent in this buildpack are prefixed with DD_, except for these:

• ENABLE_HEROKU_POSTGRES
• POSTGRES_URL_VAR
• ENABLE_HEROKU_REDIS
• REDIS_URL_VAR

This is unnecessarily confusing and a little bit risky, because these environment variables are also shared with the running application. I think it would be better if they were named something like:

• DD_HEROKU_POSTGRES_ENABLED
• DD_HEROKU_POSTGRES_URL_VAR
• DD_HEROKU_REDIS_ENABLED
• DD_HEROKU_REDIS_URL_VAR

That way, they are grouped together with other Datadog variables.

[arapulido]

Thanks for the feedback @nertzy . This has been now changed in the latest release of the buildpack.