[IMPROVEMENT] Add proxy for /api/v1/labels

[kfdm]

Support for Grafana adhoc filters

[codecov-io]

Codecov Report

Merging #140 into master will decrease coverage by 0.18%.
The diff coverage is 20%.

@@            Coverage Diff             @@
##           master     #140      +/-   ##
==========================================
- Coverage   46.68%   46.49%   -0.19%     
==========================================
  Files          40       40              
  Lines        2455     2469      +14     
==========================================
+ Hits         1146     1148       +2     
- Misses       1309     1321      +12

Impacted Files	Coverage Δ
promgen/proxy.py	29.93% <20%> (-1.54%)	⬇️

---

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 4bb287c...f69a60f. Read the comment docs.