node: fix block metric label

[libotony]

Description

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] New and existing E2E tests pass locally with my changes
• [x] Any dependent changes have been merged and published in downstream modules
• [x] I have not added any vulnerable dependencies to my code

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 62.59%. Comparing base (5fb5896) to head (27e2efe).

Files	Patch %	Lines
cmd/thor/node/node.go	0.00%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #762 +/- ## ======================================= Coverage 62.59% 62.59% ======================================= Files 199 199 Lines 18093 18093 ======================================= Hits 11326 11326 Misses 5685 5685 Partials 1082 1082 ```

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