Metadata API: Add simple threshold validation

[MVrachev]

Fixes #1439

Description of the changes being introduced by the pull request:

Probably there could be future API calls that modify "threshold"
to a new value, but the problem is we don't have a clear idea
if they would exist and what exactly they will do.
That's why it makes sense to validate against the potential problems
we can imagine - in this case, is passing a threshold below 1.

Signed-off-by: Martin Vrachev [email protected]

Please verify and check that the pull request fulfills the following
requirements:

• The code follows the Code Style Guidelines
• Tests have been added for the bug fix or new feature
• Docs have been added for the bug fix or new feature

[jku]

looks reasonable to me (spec doesn't very explicitly make this requirement but it seem logical). Doing a value check like this also implicitly checks that we have a type we can compare with <>. This seems like good enough typing to me.