Flag duplicate friendly_name for metrics

[mikewilli]

This is likely a mistake and we should flag it in CI if possible.

Example:

metric-hub/definitions/firefox_desktop.toml

Lines 1174 to 1188 in 97ca15d

	[metrics.newtab_any_searches]
	data_source = "newtab_clients_daily"
	select_expression = '{{agg_any("searches > 0")}}'
	friendly_name = "Any Newtab Searches"
	description = """
	Client performed any Newtab Handoff searches during the experiment
	"""
	[metrics.newtab_gt4_searches]
	data_source = "newtab_clients_daily"
	select_expression = 'COALESCE(CASE WHEN SUM(searches) > 4 THEN 1 ELSE 0 END, 0)'
	friendly_name = "Any Newtab Searches"
	description = """
	Client performed any Newtab Handoff searches during the experiment
	"""