perf(tags): cache tag strings; hoist per-call regexes to LazyLock

[duncanista]

Jira: none yet — add before marking ready.

DRAFT — stacked on #1271 (jordan.gonzalez/cold-start-instrumentation/feature). Review/merge the base PR first; this targets that branch, not main.

Overview

Two small cold-start / hot-path cleanups. Behavior is unchanged — same tag set, order, format, and values; same metrics. Only redundant work is removed.

1. Cache Lambda tag representations (tags/lambda/tags.rs, tags/provider.rs)

Lambda::get_tags_vec, get_tags_string, and get_function_tags_map previously re-iterated the tag map and re-ran format!("{k}:{v}") (and join(",")) on every call. These are called repeatedly during init (start_dogstatsd builds the enrichment-tag string/vec) and per-trace (trace_processor, log processor), so the same string was rebuilt many times over.

The tag set is immutable after construction, so the Vec<String>, joined String, and _dd.tags.function map are now computed once in Lambda::new_from_config and stored. The getters return the cached values, making repeated reads O(1). A get_tags_string() returning &str was added so Provider::get_tags_string() clones the cached string instead of re-joining the vec.

2. Hoist per-call regexes to LazyLock (proc/mod.rs)

get_fd_max_data_from_path and get_threads_max_data_from_path called Regex::new(...) on every invocation (every fd/threads metrics sample). Both patterns are static literals, so they are now compiled once via LazyLock<Regex> (using .expect("valid static regex")).

trace_processor::span_matches_tag_regex — left unchanged. Its pattern comes from per-call user config (apm_filter_tags_regex_reject), not a static literal, so it cannot be hoisted to a LazyLock without changing behavior. Left as-is intentionally.

Testing

• cargo fmt clean; cargo clippy --bin bottlecap --no-deps clean (only the pre-existing buf_redux/multipart future-incompat warning remains; pedantic/unwrap_used denied — satisfied, only .expect() used).
• cargo test --lib for proc::tests, tags::lambda::tags::tests, tags::provider::tests passes. (Two test_get_function_tags_map* cases are a pre-existing multi-threaded env-var-race flake — they also fail on the unmodified base branch and pass single-threaded / in isolation; not caused by this change.)

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