Customize SDK metrics for Dex

[longquanzheng]

Summary

• add runtime workflow and activity metric providers to registration options
• propagate workflow flow types through private activity headers and enrich regular/local activity metrics
• rewrite Temporal SDK metrics into the Dex metric namespace with stable label keys
• raise the fork's minimum Go version to 1.26

Rationale and user impact

Dex needs workflow and activity metrics labeled by runtime flow, step, subflow, and RPC types without coupling this SDK fork to Dex protobuf schemas. Providers keep schema knowledge in Dex while the SDK owns decoding reuse, propagation, failure handling, and metric naming. Existing temporal_* dashboards and alerts must migrate to the documented dex_* metric names.

Validation

• go run . check
• go run . unit-test
• go run . integration-test -run 'TestIntegrationSuite/(TestDexMetricsProvidersAndNames|TestDexMetricsProviderPanicsSkipBusinessFunctions)'
• WORKFLOW_CACHE_SIZE=0 go run . integration-test -run 'TestIntegrationSuite/(TestDexMetricsProvidersAndNames|TestDexMetricsProviderPanicsSkipBusinessFunctions)'
• go test ./internal/... -run TestDex
• go test -run TestReplayTestSuite/TestReplayLegacyActivityHeaderWithDexFlowTypeProvider ./test/replaytests