test: add unique identifier for dynatrace_log_metrics metric key

The metric key for `dynatrace_log_metrics` has to be unique.
When running tests in parallel or encountering an error during destroy/cleanup, the metric key may already exist. So, adding `#name#` to the key makes it unique.

Author: Kirdock

dynatrace/api/builtin/logmonitoring/schemalesslogmetric/testdata/terraform/example_a.tf

@@ -1,8 +1,8 @@
 resource "dynatrace_log_metrics" "#name#" {
   enabled           = true
   dimensions        = ["dt.os.type", "dt.entity.process_group"]
-  key               = "log.terraformexample3"
+  key               = "log.#name#"
   measure           = "ATTRIBUTE"
   measure_attribute = "dt.entity.host"
   query             = "matchesPhrase(content, \"terratest\")"
-}
+}