Okhttp Headers performance improvements.

OkHttp performance has been lackluster for a while, likely due to it not being easy to benchmark. (especially on android!). It does some extra validation we don't need (since Metadata) already does it. We should audit the okhttp path and add some benchmarks in order to measure improvement.

Start by locating the OkHttp path and any existing benchmarking infrastructure in the repository. Audit the path for redundant validation and add benchmarks that measure the relevant performance before and after the changes, including Android if supported by the existing setup.

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