Provide get_started statistic

[stormshield-fabs]

The current get_* metrics only account for completed operations, so it's not possible to know how many tasks are waiting for a connection.

This new counter get_started is incremented before we start waiting in get, allowing the user to monitor the queue size by computing the difference with the other get_* statistics.

[codecov]

Codecov Report

❌ Patch coverage is 45.45455% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.27%. Comparing base (b34c071) to head (ab0052b).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
bb8/src/api.rs	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #265      +/-   ##
==========================================
- Coverage   80.88%   80.27%   -0.61%     
==========================================
  Files           6        6              
  Lines         633      644      +11     
==========================================
+ Hits          512      517       +5     
- Misses        121      127       +6     

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