GH-2229: Ensure that BROWSER_INFO_READY before using BROWSER_INFO

[wlycdgr]

Ensure that BROWSER_INFO_READY has resolved before using BROWSER_INFO in Metrics#_buildMetricsURL. The check in Metrics#_getProductID is currently redundant but should make it easier to avoid introducing a regression if we later call this method from somewhere other than _buildMetricsURL

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