Removed redundant memory allocations in parsers

[vlad-diachenko]

What this PR does / why we need it:
This implementation reuses errors instead of creating a new one for each line that is not a valid JSON in JSONParser and Unpack parser. These errors are used as boolean flags, we just check if an error exists and if so, we add an additional label to the line with another error message.

Which issue(s) this PR fixes:
Fixes #5310

Special notes for your reviewer:

benchstat old new                                                                                              
name                                             old time/op    new time/op    delta
_Parser/json/no_labels_hints-16                    3.83µs ± 1%    3.87µs ± 6%      ~     (p=0.587 n=5+5)
_Parser/json/labels_hints-16                       2.87µs ± 1%    2.87µs ± 2%      ~     (p=0.690 n=5+5)
_Parser/not_json_line/no_labels_hints-16            247ns ± 1%      38ns ±22%   -84.70%  (p=0.008 n=5+5)
_Parser/not_json_line/labels_hints-16               244ns ± 1%      40ns ± 5%   -83.48%  (p=0.008 n=5+5)
_Parser/unpack/no_labels_hints-16                  1.01µs ± 2%    1.10µs ± 5%    +8.12%  (p=0.008 n=5+5)
_Parser/unpack/labels_hints-16                     1.02µs ± 2%    1.08µs ± 2%    +5.52%  (p=0.008 n=5+5)
_Parser/not_json_line_unpack/no_labels_hints-16     250ns ± 5%      34ns ± 1%   -86.31%  (p=0.016 n=5+4)
_Parser/not_json_line_unpack/labels_hints-16        248ns ± 2%      32ns ± 3%   -87.13%  (p=0.008 n=5+5)
_Parser/logfmt/no_labels_hints-16                  2.11µs ± 4%    2.07µs ± 1%      ~     (p=0.206 n=5+5)
_Parser/logfmt/labels_hints-16                     2.09µs ± 3%    2.10µs ± 1%      ~     (p=0.548 n=5+5)
_Parser/regex_greedy/no_labels_hints-16            4.97µs ± 1%    4.54µs ± 3%    -8.70%  (p=0.008 n=5+5)
_Parser/regex_greedy/labels_hints-16               4.64µs ± 3%    4.47µs ± 2%    -3.61%  (p=0.008 n=5+5)
_Parser/regex_status_digits/no_labels_hints-16      331ns ± 1%     332ns ± 2%      ~     (p=1.000 n=5+5)
_Parser/regex_status_digits/labels_hints-16         337ns ± 3%     327ns ± 2%      ~     (p=0.095 n=5+5)
_Parser/pattern/no_labels_hints-16                  192ns ± 1%     186ns ± 4%      ~     (p=0.151 n=5+5)
_Parser/pattern/labels_hints-16                     161ns ± 1%     153ns ± 1%    -4.61%  (p=0.008 n=5+5)

name                                             old alloc/op   new alloc/op   delta
_Parser/json/no_labels_hints-16                      656B ± 0%      656B ± 0%      ~     (all equal)
_Parser/json/labels_hints-16                         512B ± 0%      512B ± 0%      ~     (all equal)
_Parser/not_json_line/no_labels_hints-16            48.0B ± 0%      0.0B       -100.00%  (p=0.008 n=5+5)
_Parser/not_json_line/labels_hints-16               48.0B ± 0%      0.0B       -100.00%  (p=0.008 n=5+5)
_Parser/unpack/no_labels_hints-16                    537B ± 0%      537B ± 0%      ~     (all equal)
_Parser/unpack/labels_hints-16                       537B ± 0%      537B ± 0%      ~     (all equal)
_Parser/not_json_line_unpack/no_labels_hints-16     48.0B ± 0%      0.0B       -100.00%  (p=0.008 n=5+5)
_Parser/not_json_line_unpack/labels_hints-16        48.0B ± 0%      0.0B       -100.00%  (p=0.008 n=5+5)
_Parser/logfmt/no_labels_hints-16                    336B ± 0%      336B ± 0%      ~     (all equal)
_Parser/logfmt/labels_hints-16                       336B ± 0%      336B ± 0%      ~     (all equal)
_Parser/regex_greedy/no_labels_hints-16              194B ± 0%      193B ± 0%      ~     (p=0.167 n=5+5)
_Parser/regex_greedy/labels_hints-16                 194B ± 0%      194B ± 0%      ~     (all equal)
_Parser/regex_status_digits/no_labels_hints-16      52.0B ± 0%     52.0B ± 0%      ~     (all equal)
_Parser/regex_status_digits/labels_hints-16         52.0B ± 0%     52.0B ± 0%      ~     (all equal)
_Parser/pattern/no_labels_hints-16                  35.0B ± 0%     35.0B ± 0%      ~     (all equal)
_Parser/pattern/labels_hints-16                     32.0B ± 0%     32.0B ± 0%      ~     (all equal)

name                                             old allocs/op  new allocs/op  delta
_Parser/json/no_labels_hints-16                      46.0 ± 0%      46.0 ± 0%      ~     (all equal)
_Parser/json/labels_hints-16                         39.0 ± 0%      39.0 ± 0%      ~     (all equal)
_Parser/not_json_line/no_labels_hints-16             2.00 ± 0%      0.00       -100.00%  (p=0.008 n=5+5)
_Parser/not_json_line/labels_hints-16                2.00 ± 0%      0.00       -100.00%  (p=0.008 n=5+5)
_Parser/unpack/no_labels_hints-16                    17.0 ± 0%      17.0 ± 0%      ~     (all equal)
_Parser/unpack/labels_hints-16                       17.0 ± 0%      17.0 ± 0%      ~     (all equal)
_Parser/not_json_line_unpack/no_labels_hints-16      2.00 ± 0%      0.00       -100.00%  (p=0.008 n=5+5)
_Parser/not_json_line_unpack/labels_hints-16         2.00 ± 0%      0.00       -100.00%  (p=0.008 n=5+5)
_Parser/logfmt/no_labels_hints-16                    16.0 ± 0%      16.0 ± 0%      ~     (all equal)
_Parser/logfmt/labels_hints-16                       16.0 ± 0%      16.0 ± 0%      ~     (all equal)
_Parser/regex_greedy/no_labels_hints-16              2.00 ± 0%      2.00 ± 0%      ~     (all equal)
_Parser/regex_greedy/labels_hints-16                 2.00 ± 0%      2.00 ± 0%      ~     (all equal)
_Parser/regex_status_digits/no_labels_hints-16       2.00 ± 0%      2.00 ± 0%      ~     (all equal)
_Parser/regex_status_digits/labels_hints-16          2.00 ± 0%      2.00 ± 0%      ~     (all equal)
_Parser/pattern/no_labels_hints-16                   2.00 ± 0%      2.00 ± 0%      ~     (all equal)
_Parser/pattern/labels_hints-16                      1.00 ± 0%      1.00 ± 0%      ~     (all equal)

[cyriltovena]

LGTM