Werk #7296: oracle_dataguard_stats: Add missing metric definitions

Component	Checks & agents
Title	oracle_dataguard_stats: Add missing metric definitions
Date	May 2, 2019
Level	Trivial Change
Class	Bug Fix
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	1.6.0b1 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME)

CMK-1768

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