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