[core] add metrics testing

[palazzem]

What it does

• tests set_metric for valid and invalid values
• enforces that the value used in set_metric is stored as float
• invalid values can be safely discarded before sending the trace

[galdor]

Note that a cast will raise an ArgumentError.
A cleaner way would be to check if value.is_a?(Integer) || value.is_a?(Float).

[palazzem]

if nil is passed it will be a TypeError:

irb(main):001:0> Float(nil)
TypeError: can't convert nil into Float
	from (irb):1:in `Float'
	from (irb):1
	from .../versions/2.3.1/bin/irb:11:in `<main>'

The reason to use the Float(value) is that a string may be converted to float, as we're doing in the Python implementation. I think it was already discussed before about why we should at least try to make it work.

[galdor]

Ok then.

[ufoot]

👍