API client: make http reads more efficient

[bboreham]

Replace io.ReadAll with bytes.Buffer.ReadFrom.
Both need to resize a buffer until they have finished reading; the former increases by 1.25x each time while the latter uses 2x.

(I considered using ContentLength to size the buffer, but empirically Prometheus does not set it on larger responses.)

Also added a benchmark to demonstrate the benefit:

name             old time/op    new time/op    delta
Client/4KB-8       35.9µs ± 4%    35.3µs ± 3%     ~     (p=0.310 n=5+5)
Client/50KB-8      83.1µs ± 8%    69.5µs ± 1%  -16.37%  (p=0.008 n=5+5)
Client/1000KB-8     891µs ± 6%     750µs ± 0%  -15.83%  (p=0.016 n=5+4)
Client/2000KB-8    1.74ms ± 2%    1.35ms ± 1%  -22.72%  (p=0.008 n=5+5)

name             old alloc/op   new alloc/op   delta
Client/4KB-8       20.2kB ± 0%    20.4kB ± 0%   +1.26%  (p=0.008 n=5+5)
Client/50KB-8       218kB ± 0%     136kB ± 0%  -37.65%  (p=0.008 n=5+5)
Client/1000KB-8    5.88MB ± 0%    2.11MB ± 0%  -64.10%  (p=0.008 n=5+5)
Client/2000KB-8    11.7MB ± 0%     4.2MB ± 0%  -63.93%  (p=0.008 n=5+5)

name             old allocs/op  new allocs/op  delta
Client/4KB-8         75.0 ± 0%      72.0 ± 0%   -4.00%  (p=0.008 n=5+5)
Client/50KB-8         109 ± 0%        98 ± 0%  -10.09%  (p=0.079 n=4+5)
Client/1000KB-8       617 ± 0%       593 ± 0%   -3.89%  (p=0.008 n=5+5)
Client/2000KB-8     1.13k ± 0%     1.09k ± 0%   -3.27%  (p=0.008 n=5+5)

[kakkoyun]

LGTM. Thanks for the clear, detailed explanation and benchmarks.