Exclude HTTP/2 pseudo-headers from sanitization

[trentm]

(This separates out the HTTP/2 pseudo-header discussion on https://github.com/elastic/apm/pull/573. See some earlier discussion on options there.)

This is to avoid conflict between the ':authority' HTTP/2 pseudo-header and the addition of *auth* to the sanitize_field_names default patterns in #573.

The Node.js HTTP/2 core lib includes the HTTP/2 pseudo-headers in $requestObject.headers and $responseObject.headers, so its normal capturing of headers includes them. Are other agents that have HTTP/2 support the same or different?

This is a small enhancement

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#### Build stats * Start Time: 2022-01-03T04:19:00.538+0000 * Duration: 3 min 17 sec * Commit: 7079b7867bbebb3cccd68c7a845053fdcec6aeb7