[2.3.0] Checkmk Werk 16204 created: metrics: Fix Internal Server Error when decimal timestamps are provided

Title: metrics: Fix Internal Server Error when decimal timestamps are provided Class: fix Compatible: compat Component: rest-api Date: 1700216654 Edition: cre Level: 1 Version: 2.3.0b1 Prior to this werk, when a timestamp contained decimal values the endpoint would return status 500 (Internal server error). This change fixes that and now it returns 400 (Bad Request) and a brief explanation of the error.