fix: update prometheus label API usage for prometheus 0.14

[ayushsingh82]

Summary

prometheus 0.14 tightened with_label_values / get_metric_with_label_values / remove_label_values to require a single uniform label type across the slice. Several call sites mixed &String (from .to_string()) with &str constants (TAP_VERSION / TAP_V2) in the same array, which no longer type-checks. This currently breaks CI on the pending prometheus 0.14.0 bump (#1027).

Also fixes one deprecated warning (LabelPair::get_value() → .value()) that would otherwise fail the workspace's cargo clippy -D warnings gate once the above compiles.

Changes

• crates/tap-agent/src/agent/sender_allocation.rs: 4 call sites normalized to &str via .as_str()
• crates/tap-agent/src/agent/sender_account.rs: 7 call sites (incl. 3 in tests) normalized the same way
• crates/service/src/middleware/prometheus_metrics.rs: replaced deprecated get_value() with value()

Test plan

• cargo check --workspace --all-targets --all-features — compiles clean
• cargo clippy --workspace --all-features --all-targets -- -A dead-code -D warnings — clean, matching CI's exact invocation
• cargo fmt --check — no diff

Should unblock #1027 (prometheus 0.14.0) once rebased on top of, or merged alongside, this fix.

[ayushsingh82]

cc @MoonBoi9001 — same theme as #1080: this one fixes 11 with_label_values/get_metric_with_label_values/remove_label_values call sites in tap-agent (sender_allocation.rs, sender_account.rs) that mixed &String and &str labels, which prometheus 0.14 no longer allows. Also fixed one deprecated-API warning in indexer-service-rs that would've failed the clippy -D warnings gate. Unblocks the pending prometheus 0.14.0 bump (#1027).