Adding tags to Circuit objects

[kjsatz]

Is your feature request related to a use case or problem? Please describe.
I'm passing around a cirq.FrozenCircuit to take data and would like to have some descriptive information about the circuit attached to it, which other entities can access to help them understand key information about the circuits they ran.

Describe the solution you'd like
Attach metadata "tags" to Circuit objects, similar to Operation.with_tags.

[optional] Describe alternatives/workarounds you've considered
It descends into hacks, like wrapping the circuit in a CircuitOperation and tagging that, or subclassing cirq.FrozenCircuit.

[optional] Additional context (e.g. screenshots)

What is the urgency from your perspective for this issue? Is it blocking important work?
I hesitate to actually call this "P0" although I would use it immediately for something fairly important and useful.

P0 - this should land no later than a week