fix: cap distinct user-agents per endpoint in GrpcUsageTracker - #26778

  2 102 files  ±0    2 102 suites  ±0   1h 59m 20s ⏱️ +38s
173 675 tests ±0  173 622 ✅ ±0  53 💤 ±0  0 ❌ ±0 
182 427 runs  ±0  182 374 ✅ ±0  53 💤 ±0  0 ❌ ±0 

Results for commit eb695f6. ± Comparison against earlier commit b7a4df6.