warn on missing unit for duration/size config vars, require unit on new config vars

[trentm]

There was a discussion recently and suggestion from @felixbarny that the APM agents make the units on time/duration config vars (e.g. metricsInterval) required instead of optional. The history there is that some Java agent config vars that were later shared with other agents (e.g. span_frames_min_duration) used to have the units in the name (e.g. span_frames_min_duration_ms). The switch to drop the units in the name added support for units in the value, but for backward compatibility supported default units.

The problem: There are some cases where the user can get surprised:

• different default units between agents (e.g. the recently added spanFramesMinDuration in the node.js agent defaults to seconds with no units, but the default is milliseconds for other agents)
• the default units are not obvious to the user doing the configuration

Making the units required is obviously a breaking change, so would need to wait for a major version bump.

(See also the related #2120 for handling invalid config values.)

[trentm]

There was an earlier issue for this as well: #1504

[trentm]

I propose to avoid a breaking change here. In 4.x we would:

• add a warning for duration and size options that don't provide an explicit unit
• add support for and make any new duration and size options have the unit be required.

[trentm]

This was handled in #3553 and #3544 on the "dev/4.x" branch, which will be the active 4.x release branch soonish.