Add function to report metrics for a HTTP.Connection.Pool

[nickrobinson251]

• requires Add functions to count object in use and objects in pool JuliaServices/ConcurrentUtilities.jl#29

[codecov-commenter]

Codecov Report

Merging #1116 (8ab7921) into master (352bca2) will decrease coverage by 0.12%.
Report is 2 commits behind head on master.
The diff coverage is 62.96%.

@@            Coverage Diff             @@
##           master    #1116      +/-   ##
==========================================
- Coverage   82.67%   82.55%   -0.12%     
==========================================
  Files          32       32              
  Lines        3053     3073      +20     
==========================================
+ Hits         2524     2537      +13     
- Misses        529      536       +7     

Files	Coverage Δ
src/Connections.jl	83.16% <62.96%> (-1.71%)	⬇️

... and 1 file with indirect coverage changes

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