Allow setting metadata before registering to Eureka

[jianfeiliao]

Eureka has problem to replicate metadata when there are multiple instances. By setting the metadata before the registration, and only send the metadata once in one POST call, we can avoid the corruption to the metadata.

I worked with @cquinn on this, and this is the approach he used in Netflix.

[cquinn]

Eureka doesn't support sub-record syncing, so with fargo's round-robin server selection, and the use of post-registration metadata PUTing, we get corrupted metadata as it is written piecemeal to different servers and then the last one stomps the others. Allowing pre-registration metadata setting allows registration to work like it does at Netflix: it's all set once before the POST, and never updated later.

Fixing the server selection is another approach, but this fix is better all around for metadata handling, and makes server selection less critical.

The original Add* method should still work the same way, and is fairly safe if only one item is set between sync windows.

[ryansb]

I wasn't aware that the round-robin led to update-stomping. I'll review this ASAP.

[ryansb]

This change looks good to me. 👍

[tysonstewart]

Looks great. Thank you!