docs: expand metric mapping examples

[ihopenre-eng]

Fixes #82.

@pedro-stanaka, this expands the mapping documentation with Graphite-specific examples and behavior.

Summary

• separate glob and regular expression mapping examples
• document glob precedence, regex ordering, and capture group references
• compare default unmatched handling, strict matching, a diagnostic catch-all, and an explicit drop rule
• correct the existing sanitized metric name example
• call out the cardinality cost of retaining original names as labels

Verification

• validated the regex, catch-all, and drop YAML snippets with MetricMapper.InitFromYAMLString and GetMapping
• go test ./cmd/graphite_exporter ./collector
• go test ./e2e
• go vet ./cmd/graphite_exporter ./collector ./e2e