chore(flags): more metrics

[dmarticus]

Problem

Spinning out from this convo: https://posthog.slack.com/archives/C07Q2U4BH4L/p1743725916325309

[greptile-apps]

PR Summary

Added granular timing metrics for more precise flag evaluation performance tracking.

• Updated rust/feature-flags/src/metrics/metrics_consts.rs to include FLAG_GET_INDIVIDUAL_FLAG_TIME and FLAG_EVALUATE_STATIC_COHORTS_TIME.
• Modified rust/feature-flags/src/flags/flag_matching.rs to start new timers in the evaluation loop, labeling FLAG_GET_INDIVIDUAL_FLAG_TIME with the flag key.
• Introduced a new timer for evaluating static cohorts to isolate performance data.
• Noted potential instrumentation overhead increase; monitor overall performance post-deployment.

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[haacked]

👍

[posthog-bot]

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