Log how many nodes/channels we have synced when we try to route

[TheBlueMatt]

A common issue in LN is not having fully synced the network graph when we attempt to send a payment. This should be improved substantially with gossip v1.5, but for now we should improve our debugability by logging how many nodes we have in our graph when we attempt to find a route.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 89.79%. Comparing base (1237abf) to head (7f16039).

Files	Patch %	Lines
lightning/src/routing/router.rs	0.00%	1 Missing and 1 partial ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3069      +/-   ##
==========================================
- Coverage   89.83%   89.79%   -0.04%     
==========================================
  Files         116      116              
  Lines       96466    96467       +1     
  Branches    96466    96467       +1     
==========================================
- Hits        86656    86622      -34     
- Misses       7263     7288      +25     
- Partials     2547     2557      +10     

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[tnull]

LGTM

Trivial, so no need for a second reviewer.