feat(l2-watcher): add nil row consumption block metric

[colinlyguo]

Purpose or design rationale of this PR

This PR adds a metric to track the number of times that l2 watcher fetches a block with nil row consumption:
When it's added, there are two cases:
(i) this field is temporarily missed in the internal node, and it will be recovered shortly after async ccc fills it.
(ii) a bug case: either the signer or internal node generates a block without row consumption.

PR title

Your PR title must follow conventional commits (as we are doing squash merge for each PR), so it must start with one of the following types:

• feat: A new feature

Deployment tag versioning

Has tag in common/version.go been updated or have you added bump-version label to this PR?

• Yes

Breaking change label

Does this PR have the breaking-change label?

• No, this PR is not a breaking change

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 51.61%. Comparing base (246bf38) to head (949487a).

Files with missing lines	Patch %	Lines
rollup/internal/controller/watcher/l2_watcher.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1518      +/-   ##
===========================================
+ Coverage    51.60%   51.61%   +0.01%     
===========================================
  Files          155      155              
  Lines        12775    12780       +5     
===========================================
+ Hits          6593     6597       +4     
- Misses        5608     5609       +1     
  Partials       574      574              

Flag	Coverage Δ
rollup	55.40% <80.00%> (+0.02%)	⬆️

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