Unify metric and trace tag filtering interface

[JamesGuthrie]

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As of 0.5.0 tracing tags could be used inside where clauses as if they were regular JSON, and optimisations happen transparently whenever possible (see #189). Yet metrics tags still rely on a non-standard DSL. Moving forward, we would like both metrics and tracing tags to converge on a standard JSONB-like interface.

[sumerman]

As an aside, optimizations in the new interface only support -> operator, which could be a nuisance for human users, we should consider supporting ->> as well.