support for specifying precision when writing metric

[wmclendon]

hello,

we have scenario where we receive timestamp in ms specificity, and would like to specify the precision as ms in the write to influx so it is stored as proper value. As of now it shows up as a 1970 timestamp due to time being in ms accuracy but influx interpreting it as ns accuracy. I see in the code this is not presently implemented. i'm not sure what is required to implement, if it not overly complicated I may be able to do it.

in the meantime I guess we can convert our time to ns from ms and then write with this client, but it would be great to not to have to do that.

Thanks.

[gusutabopb]

This has been asked before, but implementation is non-trivial and may affect other parts of functionality. See #13 for details.