Tracking issue: group budgets

[flyingmutant]

Multi-tenancy at the level of metric groups, not individual metrics.

• groups are simple prefixes (without trailing _)
• groups must be fully disjoint (no prefixes of prefixes etc.)
• default group always exists (without DB), and has different semantics for budget and access
• default group has fixed weight of 100
• group has separate fields: optional<number> for budget weight and protected bits (separate read and write, for e.g. common)
• no UI changes except for group admin page

TODO:

• create separate task for "fair key" in metrics (like common_, sys_ etc.)
• require global admin bit to change metric group via renaming (because of access concerns)
• builtin metric to track space used for each group