[signaltometricsconnector] Add gate for default error_mode ignore

[MukundaKatta]

Description

Adds an alpha feature gate for the signaltometrics connector to switch the default error_mode from propagate to ignore, matching the rollout requested in #48419. The existing default remains propagate until connector.signaltometrics.defaultErrorModeIgnore is enabled.

Also updates the config docs/schema and covers both default paths in a factory test.

Link to tracking issue

Fixes #48419

Testing

• go test ./... from connector/signaltometricsconnector
• git diff --check

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