[exporter/splunkhec] asynchronous HTTP request consumption

[atoulme]

Description:
only return the buffer to the pool after the HTTP request has been closed by the HTTP client

Link to tracking Issue:
Fixes #34255

Testing:
Tests pass. Performance tests show a large performance hit.

                                      │  before.txt  │               after.txt               │
                                      │    sec/op    │    sec/op     vs base                 │
_pushLogData_10_10_1024-10              91.11µ ± ∞ ¹   89.49µ ± ∞ ¹        ~ (p=1.000 n=1) ²
_pushLogData_10_10_8K-10                76.62µ ± ∞ ¹   86.14µ ± ∞ ¹        ~ (p=1.000 n=1) ²
_pushLogData_10_10_2M-10                79.62µ ± ∞ ¹   88.56µ ± ∞ ¹        ~ (p=1.000 n=1) ²
_pushLogData_10_200_2M-10               1.528m ± ∞ ¹   1.707m ± ∞ ¹        ~ (p=1.000 n=1) ²
_pushLogData_100_200_2M-10              14.37m ± ∞ ¹   15.10m ± ∞ ¹        ~ (p=1.000 n=1) ²
_pushLogData_100_200_5M-10              14.18m ± ∞ ¹   16.03m ± ∞ ¹        ~ (p=1.000 n=1) ²
_pushLogData_compressed_10_10_1024-10   597.9µ ± ∞ ¹   770.1µ ± ∞ ¹        ~ (p=1.000 n=1) ²
_pushLogData_compressed_10_10_8K-10     136.3µ ± ∞ ¹   264.0µ ± ∞ ¹        ~ (p=1.000 n=1) ²
_pushLogData_compressed_10_10_2M-10     136.1µ ± ∞ ¹   254.7µ ± ∞ ¹        ~ (p=1.000 n=1) ²
_pushLogData_compressed_10_200_2M-10    3.064m ± ∞ ¹   3.547m ± ∞ ¹        ~ (p=1.000 n=1) ²
_pushLogData_compressed_100_200_2M-10   30.55m ± ∞ ¹   31.86m ± ∞ ¹        ~ (p=1.000 n=1) ²
_pushLogData_compressed_100_200_5M-10   30.30m ± ∞ ¹   31.35m ± ∞ ¹        ~ (p=1.000 n=1) ²
geomean                                 1.152m         1.389m        +20.64%
¹ need >= 6 samples for confidence interval at level 0.95
² need >= 4 samples to detect a difference at alpha level 0.05

                                      │  before.txt   │                after.txt                 │
                                      │     B/op      │      B/op       vs base                  │
_pushLogData_10_10_1024-10              73.16Ki ± ∞ ¹    75.62Ki ± ∞ ¹         ~ (p=1.000 n=1) ²
_pushLogData_10_10_8K-10                63.82Ki ± ∞ ¹    84.78Ki ± ∞ ¹         ~ (p=1.000 n=1) ²
_pushLogData_10_10_2M-10                63.27Ki ± ∞ ¹   105.46Ki ± ∞ ¹         ~ (p=1.000 n=1) ²
_pushLogData_10_200_2M-10               1.257Mi ± ∞ ¹    1.997Mi ± ∞ ¹         ~ (p=1.000 n=1) ²
_pushLogData_100_200_2M-10              12.60Mi ± ∞ ¹    18.51Mi ± ∞ ¹         ~ (p=1.000 n=1) ²
_pushLogData_100_200_5M-10              12.68Mi ± ∞ ¹    24.51Mi ± ∞ ¹         ~ (p=1.000 n=1) ²
_pushLogData_compressed_10_10_1024-10   2.417Mi ± ∞ ¹    3.191Mi ± ∞ ¹         ~ (p=1.000 n=1) ²
_pushLogData_compressed_10_10_8K-10     64.24Ki ± ∞ ¹   860.77Ki ± ∞ ¹         ~ (p=1.000 n=1) ²
_pushLogData_compressed_10_10_2M-10     64.40Ki ± ∞ ¹   860.70Ki ± ∞ ¹         ~ (p=1.000 n=1) ²
_pushLogData_compressed_10_200_2M-10    1.257Mi ± ∞ ¹    2.045Mi ± ∞ ¹         ~ (p=1.000 n=1) ²
_pushLogData_compressed_100_200_2M-10   12.54Mi ± ∞ ¹    13.42Mi ± ∞ ¹         ~ (p=1.000 n=1) ²
_pushLogData_compressed_100_200_5M-10   12.54Mi ± ∞ ¹    13.42Mi ± ∞ ¹         ~ (p=1.000 n=1) ²
geomean                                 848.1Ki          1.671Mi        +101.75%
¹ need >= 6 samples for confidence interval at level 0.95
² need >= 4 samples to detect a difference at alpha level 0.05

                                      │  before.txt  │              after.txt               │
                                      │  allocs/op   │  allocs/op    vs base                │
_pushLogData_10_10_1024-10               821.0 ± ∞ ¹    828.0 ± ∞ ¹       ~ (p=1.000 n=1) ²
_pushLogData_10_10_8K-10                 716.0 ± ∞ ¹    726.0 ± ∞ ¹       ~ (p=1.000 n=1) ²
_pushLogData_10_10_2M-10                 709.0 ± ∞ ¹    721.0 ± ∞ ¹       ~ (p=1.000 n=1) ²
_pushLogData_10_200_2M-10               14.02k ± ∞ ¹   14.04k ± ∞ ¹       ~ (p=1.000 n=1) ²
_pushLogData_100_200_2M-10              140.0k ± ∞ ¹   140.1k ± ∞ ¹       ~ (p=1.000 n=1) ²
_pushLogData_100_200_5M-10              140.0k ± ∞ ¹   140.1k ± ∞ ¹       ~ (p=1.000 n=1) ²
_pushLogData_compressed_10_10_1024-10    785.0 ± ∞ ¹    817.0 ± ∞ ¹       ~ (p=1.000 n=1) ²
_pushLogData_compressed_10_10_8K-10      709.0 ± ∞ ¹    738.0 ± ∞ ¹       ~ (p=1.000 n=1) ²
_pushLogData_compressed_10_10_2M-10      709.0 ± ∞ ¹    738.0 ± ∞ ¹       ~ (p=1.000 n=1) ²
_pushLogData_compressed_10_200_2M-10    14.02k ± ∞ ¹   14.05k ± ∞ ¹       ~ (p=1.000 n=1) ²
_pushLogData_compressed_100_200_2M-10   140.0k ± ∞ ¹   140.1k ± ∞ ¹       ~ (p=1.000 n=1) ²
_pushLogData_compressed_100_200_5M-10   140.0k ± ∞ ¹   140.1k ± ∞ ¹       ~ (p=1.000 n=1) ²
geomean                                 6.937k         7.033k        +1.38%

[PaulBernier]

onClose is nullable, so that call could result in a panic in case no closing hook has been attached

[atoulme]

this is a costly issue. We might need to review a bit more the code and come up with a different approach.