Add SLO objective on the info metric as a label

[slok]

This PR adds the objective to the SLO info metric.

Having this as a key on the info metric would give us the ability to set the objective as a string on the Grafana dashboard and fix the rounding problem described in #131.

[codecov-commenter]

Codecov Report

Merging #172 (886f97a) into main (9bb7417) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #172      +/-   ##
==========================================
+ Coverage   78.10%   78.12%   +0.01%     
==========================================
  Files          16       16              
  Lines        1311     1312       +1     
==========================================
+ Hits         1024     1025       +1     
  Misses        233      233              
  Partials       54       54              

Impacted Files	Coverage Δ
internal/prometheus/recording_rules.go	90.09% <100.00%> (+0.04%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9bb7417...886f97a. Read the comment docs.