actx.tag() comes too late to benefit eager mode execution

For lazy mode, `actx.tag(MyTag(), 2*x+3)` works great. For eager mode, this doesn't help at all, as the result will already be computed (i.e. the kernel whose transformation would have benefited from the metadata was already executed). Is there a tagging interface that can work for both eager and lazy?

cc @alexfikl @thomasgibson @nchristensen @kaushikcfd

x-ref: inducer/meshmode#176

Start by reading the actx.tag() implementation and tracing how tagged expressions are handled in lazy and eager execution. Determine the interface needed for metadata to affect the relevant kernel before eager computation, and verify that the behavior works consistently in both modes.

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