fix: Solve [object Object] on all labels

[carboneater]

I might be a wee bit impatient.

Here's my take to solve #81

From Node REPL:

> Object.fromEntries(Object.entries(lokiLabels).map(([key,value]) => [key, value ? value.toString() : value]))
{ host: 'dev', env: 'test', key: 'value' }

> JSON.parse(JSON.stringify(lokiLabels, (key, value) => value ? value.toString() : value))
'[object Object]'

Fixes #81

I also tweaked a few related tests so there are less failing tests in a run.

[JaniAnttonen]

Not at all! Thanks for the fix and the tests, not sure if I'm proud seeing those fixtures :D