refactor(publish-metrics): change vendor attributes format for consistency

[InesNi]

Description

Changing the attributes format of the Dynatrace and New Relic reporters tracing feature (introduced in #2418 ) from object:

attributes:
  testType: load
  tool: Artillery

to array of strings:

attributes:
  - "testType:load"
  - "tool:Artillery"

for consistency reasons, as the equivalent setting for the metrics and events in those reporters uses the string format.
e.g. New Relic reporter config for metrics and events:

config:
  plugins:
    publish-metrics:
      - type: newrelic
        licenseKey: "{{ $env.NR_LICENSE_KEY }}"
        region: eu
        prefix: "artillery.publish_metrics_plugin."
        attributes:
          - "testType:LoadTest"
          - "tool:Artillery"
        event:
          accountId: "{{ $env.NR_ACCOUNT_ID }}"
          eventType: "Artillery_load_test"
          attributes:
            - "alertType:info"
            - "priority:low"