remove mention of custom metrics (#7040)
remove mention of custom metrics (#7040)

directory check doesn't have any custom metrics: https://datadoghq.atlassian.net/browse/DOCS-496
DataDog/integrations-core,DataDog/integrations-core,DataDog/integrations-core,DataDog/integrations-core,DataDog/integrations-core,DataDog/integrations-core,DataDog/integrations-core,DataDog/integrations-core,DataDog/integrations-core,DataDog/integrations-core