[PHP] Reinstate MetricsClient::SCHEMA constant
The MetricsClient::SCHEMA constant was removed in 049ea572 but it is still in use in EventLogging. That use MUST be removed first.
Reinstate the constant and mark it as deprecated.
Bug: T351294
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The MetricsClient::SCHEMA constant was removed in 049ea572 but it is still in use in EventLogging. That use MUST be removed first.
Reinstate the constant and mark it as deprecated.
Bug: T351294