[APM] Agent duration-type configuration input has incorrect/missing units

[axw]

Kibana version:

7.7.0-BC2

Elasticsearch version:

7.7.0-BC2

Describe the bug:

Agents have duration-type configurations, such as span_frames_min_duration, which support the suffixes "ms", "s", and "m". e.g. see the Java agent docs for span_frames_min_duration.

The UI provides options "s", "m", and "h".

Steps to reproduce:

1. Create agent config entry
2. Navigate to "Span frames minimum duration"
3. Click the "Select unit" drop-down, note the options

Expected behavior:

UI should allow selection of "ms", "s", and "m".

Screenshots (if relevant):

(Also note that the default is 5ms for this particular config.)

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)

[estolfo]

The UI can't handle the values 0 and -1 right now for the span_frames_min_duration field either. Both are valid values for the Ruby and Go agents.
The interface shows an error: Please specify an integer and a unit if either value is entered.