feat(bigtable): add client epoch for each call

[sushanb]

No description provided.

[bhshkh]

Have you tested this ? Do the headers contain correct value?

[mutianf]

Is there existing test that checks the request metdata? If so, can we add a test to check that the epoch time metadata is in the header?

[sushanb]

Have you tested this ? Do the headers contain correct value?

I will add a unit test but can confirm that the value is being populated via inspecting server loggs.