Deduplicate instance ids and restore target_instance in Prometheus

[mariomac]

Should fix #1124

This PR also changes the way instance IDs are composed, to avoid letting them grow too much. Now they are just Base32 hashes.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 92.85714% with 2 lines in your changes missing coverage. Please review.

Project coverage is 82.05%. Comparing base (ecc9dc1) to head (42165b4).

Files with missing lines	Patch %	Lines
pkg/internal/request/span_getters.go	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1129      +/-   ##
==========================================
+ Coverage   73.06%   82.05%   +8.99%     
==========================================
  Files         139      141       +2     
  Lines       11553    11586      +33     
==========================================
+ Hits         8441     9507    +1066     
+ Misses       2454     1556     -898     
+ Partials      658      523     -135     

Flag	Coverage Δ
integration-test	57.36% <89.28%> (+0.01%)	⬆️
k8s-integration-test	59.31% <92.85%> (+0.21%)	⬆️
oats-test	36.92% <57.14%> (+0.05%)	⬆️
unittests	52.38% <85.71%> (?)

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