out_loki: Add http_buffer_max_size option and handle HTTP 5xx errors

[drbugfinder-work]

Adds http_buffer_max_size option, default is 512K now. Purges the upstream connection in case of HTTP 5xx errors.

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Testing
Before we can approve your change; please submit the following in a comment:

• [N/A] Example configuration file for the change
• [N/A] Debug log output from testing the change

• Attached Valgrind output that shows no leaks or memory corruption was found

If this is a change to packaging of containers or native binaries then please confirm it works for all targets.

• [N/A] Run local packaging test showing all targets (including any new ones) build.
• [N/A] Set ok-package-test label to test for all targets (requires maintainer to do).

Documentation

• [N/A] Documentation required for this feature

Backporting

• [N/A] Backport to latest stable release.

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Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

[drbugfinder-work]

root@fb-loki:~/fluent-bit-loki/fluent-bit/build# valgrind --leak-check=full ./bin/flb-rt-out_loki 
==22521== Memcheck, a memory error detector
==22521== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==22521== Using Valgrind-3.15.0 and LibVEX; rerun with -h for copyright info
==22521== Command: ./bin/flb-rt-out_loki
==22521== 
Test remove_keys_remove_map...                  [ OK ]
Test labels_ra...                               [ OK ]
Test remove_keys...                             [ OK ]
Test basic...                                   [ OK ]
Test labels...                                  [ OK ]
Test label_keys...                              [ OK ]
Test line_format...                             [ OK ]
Test label_map_path...                          [ OK ]
Test float_value...                             [ OK ]
SUCCESS: All unit tests have passed.
==22521== 
==22521== HEAP SUMMARY:
==22521==     in use at exit: 0 bytes in 0 blocks
==22521==   total heap usage: 14,100 allocs, 14,100 frees, 6,743,280 bytes allocated
==22521== 
==22521== All heap blocks were freed -- no leaks are possible
==22521== 
==22521== For lists of detected and suppressed errors, rerun with: -s
==22521== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)