[ignore] Add custom metric tag dropping

[jchrostek-dd]

Please include Jira ticket in title.

Overview

Motivation/Summary

Allow customers to reduce metric tag cardinality by configuring tag keys that should be omitted from custom metrics emitted by the Lambda extension.

Changes

• Added DD_CUSTOM_METRICS_TAGS_DROP parsing as a comma-separated list of tag keys.
• Filtered configured tag keys out of the metric aggregator's extension-provided tags.
• Applied the same filtering to enhanced metric dynamic tags before submission.
• Added unit coverage for tag-drop matching, tag-provider filtering, env parsing, and enhanced metric dynamic tag filtering.

Testing

• RUSTUP_TOOLCHAIN=stable cargo fmt --all
• RUSTUP_TOOLCHAIN=stable cargo fmt --all -- --check
• git diff --check
• Attempted RUSTUP_TOOLCHAIN=stable cargo test custom_metrics_tags_drop and RUSTUP_TOOLCHAIN=stable cargo test --locked --offline custom_metrics_tags_drop, but Cargo could not fetch the serverless-components git dependency in this sandbox (403 / offline cache miss).

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Bits Dev status: ✅ Done

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