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 85.05%. Comparing base (bbc5abe) to head (3bb2707).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #224   +/-   ##
=======================================
  Coverage   85.05%   85.05%           
=======================================
  Files          91       91           
  Lines        9529     9529           
=======================================
  Hits         8105     8105           
  Misses       1424     1424           

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[coachchucksol]

lgtm

[coachchucksol]

lgtm