Bug 1906853 - add glean probe for http3 udp datagram size r=necko-rev…

…iewers,kershaw,janerik

This commit adds four Glean probes:
- http3_udp_datagram_segment_size_sent
- http3_udp_datagram_segment_size_received
- http3_udp_datagram_size_received
- http3_udp_datagram_num_segments_received

Given the performance impact tracking Glean metrics in the UDP hot path, see
https://phabricator.services.mozilla.com/D216034#7453056, this commit introduces
a sample buffer per metric.

This will enable us to measure the impact of:

- Implementation of Packetization Layer Path MTU Discovery for Datagram Transports (RFC 8899) [in Neqo](mozilla/neqo#1903)
- [Fast UDP for Firefox](https://bugzilla.mozilla.org/show_bug.cgi?id=1901292)

Differential Revision: https://phabricator.services.mozilla.com/D216034

UltraBlame original commit: 16d3f312970444dd8bc8af65629cbef7d7dbcd62

netwerk/metrics.yaml

@@ -8193,3 +8193,366 @@ com
 expires
 :
 never
+http_3_udp_datagram_segment_size_sent
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+-
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+g
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+Linux
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+/
+github
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+com
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+torvalds
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