Report custom labels set by agent admins back

[6543]

Let the server admin be able to see what custom labels an agent admin did set.

So he e.g. can debug why things are not picked up by some agent or when they pick up things they should not.

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Sponsored by Kithara Software GmbH

[woodpecker-bot]

Tearing down https://woodpecker-ci-woodpecker-pr-4141.surge.sh

[codecov]

Codecov Report

Attention: Patch coverage is 9.90099% with 91 lines in your changes missing coverage. Please review.

Project coverage is 26.46%. Comparing base (8b4d440) to head (4f43030).
Report is 35 commits behind head on main.

Files with missing lines	Patch %	Lines
pipeline/rpc/proto/woodpecker.pb.go	0.00%	72 Missing ⚠️
agent/rpc/client_grpc.go	0.00%	8 Missing ⚠️
server/grpc/server.go	0.00%	8 Missing ⚠️
...astore/migration/016_add_custom_labels_to_agent.go	57.14%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4141      +/-   ##
==========================================
- Coverage   26.48%   26.46%   -0.03%     
==========================================
  Files         375      376       +1     
  Lines       27396    27455      +59     
==========================================
+ Hits         7255     7265      +10     
- Misses      19477    19525      +48     
- Partials      664      665       +1     

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