add batchpin counter prometheus metric

[shorsher]

Adds BatchPinCounter prometheus metric, which will be incremented each time a batchpin is submitted

[codecov-commenter]

Codecov Report

Merging #334 (fa6bb94) into main (6761542) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #334   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          232       232           
  Lines        12721     12724    +3     
=========================================
+ Hits         12721     12724    +3     

Impacted Files	Coverage Δ
internal/apiserver/server.go	100.00% <100.00%> (ø)
internal/batchpin/batchpin.go	100.00% <100.00%> (ø)

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[onelapahead]

lgtm for the first metric! Thank you for getting this going @shorsher.

Next PR I think we can then start experimenting with metric labels and using histograms to time the total duration of batchpins across the DB and blockchain interactions.

For those interested, here are some good resources for learning more about Prometheus metric types and when to use histograms specifically (TL;DR pretty much always use them instead of summaries if you want to aggregate across multiple processes / pods):

• https://prometheus.io/docs/concepts/metric_types/
• https://prometheus.io/docs/practices/histograms/