Fix typo: Prometheus Metrics

What does this MR do?

Fixes typo about Prometheus Metrics

$ git grep -i 'Prometheus Metric\b' | wc -l
       1
$ git grep -i 'Prometheus Metrics' | wc -l
      86

Related issues

n/a

Author's checklist

• Apply the correct labels and milestone
• [n/a] Crosslink the document from the higher-level index
• [n/a] Crosslink the document from other subject-related docs
• [n/a] Feature moving tiers? Make sure the change is also reflected in features.yml
• [n/a] Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards