Report metric for GetObject TTFB

[arielshaqed]

Currently GetObject reports duration for the entire GET. This is obviously useful, but equally useful is GetObject time to first byte (TTFB), which is insensitive to how the client reads the data. Obviously we cannot measure actual TTFB, but e.g. measuring until we start sending data should be close enough.

Required to provide SLOs for GetObject performance - otherwise network-distant clients ruin SLIs. Client performance should be measured on clients.

[Isan-Rivkin]

@arielshaqed can you please clarify what do you mean for we start sending data?

• When data goes through us do you mean literally when client starts reading or it's good enough if we report right before the reader starts working?
• Do we need that for get_object in S3 GW as well?
• How do you expect to handle presigned requests?