metrics(blob): Add trace paths through blob module to trace Read paths

[ramin]

There have been some loose reports of "reads being slow". Though this is vague, a first step is to add traces to the blob module read codepath.

Then we can use a testing tool (such as waypoint) to perform many reads and see what traces are emitted

success:

• blob module read paths are instruments with traces
• potentially add some timing metrics too

[Wondertan]

@vgonkivs and @ramin, can we say it's closed or not?