Fix inconsistent join request counting

[bastbu]

What is being addressed

Before this PR, the join requests were counted in an inconsistent manner. After this PR, each join request is counted exactly once per gateway (independently of the deduplication strategy selected).

To validate this behavior we also add test infrastructure that can be used to unit test raising metrics.

[codecov-commenter]

Codecov Report

Merging #1465 (9ab6319) into dev (15ebf6e) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev    #1465      +/-   ##
==========================================
+ Coverage   84.86%   84.87%   +0.01%     
==========================================
  Files         226      226              
  Lines        8668     8675       +7     
==========================================
+ Hits         7356     7363       +7     
  Misses       1312     1312              

Impacted Files	Coverage Δ
...sStation/Processors/LnsProtocolMessageProcessor.cs	84.39% <ø> (-0.11%)	⬇️
...LoRaWan.NetworkServer/JoinRequestMessageHandler.cs	89.10% <100.00%> (+0.14%)	⬆️
...kSrvModule/LoRaWan.NetworkServer/MetricRegistry.cs	100.00% <100.00%> (ø)
.../LoRaWan.NetworkServer/PrometheusMetricExporter.cs	85.00% <100.00%> (ø)
...le/LoRaWan.NetworkServer/RegistryMetricExporter.cs	94.02% <100.00%> (+0.58%)	⬆️

---

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 15ebf6e...9ab6319. Read the comment docs.