server: attribute gRPC traffic to client implementation via x-user-agent

Kobe Nysrequested to merge
user-agents into master

Summary by CodeRabbit

  • New Features
    • Sends an x-user-agent header on Ark RPCs; default is bark/<version>, configurable via Config.user_agent or ServerConnectionBuilder::user_agent().
    • Improves server telemetry by adding a normalized, low-cardinality client label to gRPC metrics/traces (version stripped; unknown clients bucketed).
  • Bug Fixes
    • Connection setup now validates user-agent metadata and reports user-agent/access-token conversion failures more clearly.
  • Documentation
    • Updated unreleased changelogs describing the new header and client-attributed telemetry.
Edited by coderabbit

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