For a measure space $(X,\mathcal{A},\mu)$, the space of "observables" with respect to finite set $F$ which is endowed with counting measure on all of its subsets, is defined as follows:

$$obs (X, \mu, F)=\{f:X\to F \,|\, f  \,\,\text{is measurable}\}.$$ 

$obs(X, \mu, F)$ With metric $d$ which measures the points of difference of two functions is a metric space:

$$d(f, g)=\mu \{x|f(x) \neq g(x)\}. $$

My questions are:

1- where the term "observable" comes from and why?

2- Which connections may I expect from this correspondence, by correspondence I mean corresponding a metric space $obs(X, \mu, F)$ to a measure space $(X,\mathcal{A},\mu)$, and which connection exists?