docs: explain more aboutvalue_extractor

[sahya]

Summary of changes:

• Added clarification that value_extractor is required for distribution logs-based metrics
• Provided examples for EXTRACT(field) and REGEXP_EXTRACT(field, regex)
• Clarified the arguments for value_extractor - "field" and "regex"
• Mentioned the requirement of correct regular expression for the "regex" argument
  Highlighted the requirement of bucket_options and value_extractor for DISTRIBUTION value type logs-based metrics

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[sahya]

Related hashicorp/terraform-provider-google#14339