fix: stat DownloadPeerCount and DownloadPieceCount

[gaius-qi]

Description

• Fix DownloadPeerCount and DownloadPieceCount metrics.

Related Issue

Motivation and Context

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation Update (if none of the other choices apply)

Checklist

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

[codecov]

Codecov Report

Merging #2180 (e76a54a) into main (1037b6d) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2180      +/-   ##
==========================================
+ Coverage   48.22%   48.24%   +0.01%     
==========================================
  Files         141      141              
  Lines       19658    19668      +10     
==========================================
+ Hits         9481     9488       +7     
- Misses       9589     9592       +3     
  Partials      588      588              

Flag	Coverage Δ
Object-compatibility-e2etests	?
e2etests	?
unittests	48.24% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
scheduler/service/service_v2.go	83.80% <100.00%> (+0.16%)	⬆️

... and 1 file with indirect coverage changes

[hyy0322]

lgtm