I'm not sure that "making the graph smooth" is a good goal in itself; the 
variability you're seeing could well be a better representation of your 
input data.

Imagine that in one scrape your request durations samples were:
- 60ms
- 62ms
- 65ms
- 63ms
- 61ms

and in a later scrape they were:
- 93ms
- 85ms
- 87ms
- 89ms
- 92ms

With a histogram, all these points fall into the "50-100ms" bucket.  Both 
cases will be treated as identical (5 entries in the 50-100ms bucket), and 
hence all the variation has been lost.  Any function which derives data 
from the histogram. including quantile estimation, will give the same 
results for both scrapes.

With a summary, a streaming calculation takes place on the client side to 
return the 50th, 90th and 99th percentile values, based on the current 
value and recent history.  These percentiles adapt dynamically based on the 
input values received.  It can therefore capture those percentiles more 
accurately, tracking variations in the input data.

If you have a look at the raw logs of the put request durations which are 
being fed in, it may become clearer what's going on.

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