enhance(metrics): include route specific queue length

[ecrupper]

Helpful info for installations with multiple queue routes

[codecov]

Codecov Report

Attention: Patch coverage is 26.31579% with 14 lines in your changes missing coverage. Please review.

Project coverage is 56.56%. Comparing base (355199f) to head (25a76d8).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
api/metrics.go	0.00%	11 Missing ⚠️
queue/redis/route_length.go	62.50%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1237      +/-   ##
==========================================
- Coverage   56.57%   56.56%   -0.01%     
==========================================
  Files         628      629       +1     
  Lines       35596    35609      +13     
==========================================
+ Hits        20139    20144       +5     
- Misses      14777    14784       +7     
- Partials      680      681       +1     

Files with missing lines	Coverage Δ
queue/redis/route_length.go	62.50% <62.50%> (ø)
api/metrics.go	0.00% <0.00%> (ø)

[KellyMerrick]

lgtm