Reserved ping names

The Glean SDK reserves all ping names in send_in_pings starting with glean_.

This currently includes, but is not limited to:

• glean_client_info: metrics sent with this ping are added to every ping in its client_info section;
• glean_internal_info: metrics sent with this ping are added to every ping in its ping_info section.

Additionally, only Glean may specify all-pings. This special value has no effect in the client, but indicates to the backend infrastructure that a metric may appear in any ping.