Implement metrics blocklisting in thanos with negation pattern support

[willh-db]

Changes

• Adds a blocklist filter to the receive path that drops time series matching configurable M3-style filter rules (exact, wildcard, negation, alternation) before writing
• Rules are defined as a YAML list of filter strings via --receive.blocklist-config-file and applied before relabeling
• Supports optional hot-reload via --receive.blocklist-config-reload-timer for emergency blocking without restarts
• Exposes metrics: blocklist_dropped_series_total, blocklist_active_rules, blocklist_config_reload_total, blocklist_config_reload_err_total

Verification

• Unit tests for config parsing (empty, single, multiple rules, invalid input)
• Unit tests for filtering (exact match, wildcard, negation, alternation, multi-rule, all-blocked, none-blocked, nil filter)
• Hot-reload tests (valid update, invalid config preserves old rules)
• Integration tests in handler_test.go validating end-to-end blocklist behavior

[yuchen-db]

looks like some irrelevant commits are included. do you mind rebasing on latest db_main?