Return the name of the metric group/SLO an annotated function is part of

[gagbo]

To help diff-metrics produce a version 2 of the dataset to compare, am_list must also return whether an annotated function is part of an SLO group.

To avoid having to manually resolve variable names, the returned name will only be whatever's contained in the annotation directly. If a function is annotated as

#[autometrcs(objective = SOME_CONST_SLO)]
fn handler() {
// ...
}

then the objective_name will just be SOME_CONST_SLO instead of trying to resolve the value.

1. Not trying to track a constant from lexical analysis across a complete project lowers the difficulty of the task considerably.
2. It is impossible to actually resolve that const if the value is defined in a dependency crate.
3. It is impossible to actually resolve that const if the value is fetched from the environment running the application.