Fix Prometheus $digit escape for OTel expandconverter

[Paamicky]

Description of the issue

On EKS environments, Prometheus relabel_configs that use regex capture group references (e.g., $1, $2 in replacement fields) are silently corrupted. The OTel expandconverter treats bare $digit sequences as environment variable references and replaces them with empty strings, breaking label relabeling rules.

Description of changes

Added escapeDollarDigit to the OTel Prometheus receiver translator. Before the Prometheus config file is unmarshaled, bare $digit sequences are doubled to $$digit. The expandconverter interprets $$ as a literal $, so the values are correctly preserved as $1, $2, etc. when the collector starts.

The same pattern already existed in the Container Insights translator, this change brings Prometheus to parity.

Tests

• TestEscapeDollarDigit: 11 unit test cases covering bare $1–$9, $0, multi-digit $10, already-escaped $$1, $ followed by non-digit, mixed text, empty string
• TestPrometheusReceiverTranslatorDollarEscape: integration test using a real prometheus config with relabel_configs containing $1 and $1:$2, asserting the translator correctly escapes them to $$1 and $$1:$$2 before the expandconverter runs

License

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Requirements

Before commiting your code, please do the following steps.

1. Run make fmt and make fmt-sh
2. Run make lint

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Integration Tests

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