cranker: emit cluster name in metrics

[gzalz]

Problem
We would like to differentiate between instances of restaking cranker running against different Solana clusters but lack the ability to do so.

Solution

• Wire through the cluster arg to datapoint_info! calls as a tag

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.85%. Comparing base (12fa363) to head (1460fd7).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #224   +/-   ##
=======================================
  Coverage   84.85%   84.85%           
=======================================
  Files          91       91           
  Lines        9504     9504           
=======================================
  Hits         8065     8065           
  Misses       1439     1439           

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