Werk #12492: Fix add metric from service view to custom and forecast graphs

Component	Metrics system
Title	Fix add metric from service view to custom and forecast graphs
Date	Jun 15, 2021
Level	Trivial Change
Class	Bug Fix
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	2.1.0b1 Checkmk Pro, Checkmk Ultimate MT 2.0.0p6 Checkmk Pro, Checkmk Ultimate MT

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