Fix Service Ping payload hash key shadowing

What does this MR do and why?

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/363496.

This converts old Service Ping payload from Gitlab::UsageData to HashWithIndifferentAccess to prevent adding duplicate string and symbol keys on merge with instrumentation class metrics and overriding each other when converted to jsonb and persisted in RawUsageData.

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