new(libsinsp): add len() filter transformer

[LucaGuerra]

What type of PR is this?

Uncomment one (or more) /kind <> lines:

/kind feature

Any specific area of the project related to this PR?

Uncomment one (or more) /area <> lines:

/area libsinsp

Does this PR require a change in the driver versions?

No

What this PR does / why we need it:

This introduces a new len() transformer which acts like you'd expect:

• len(some_list) evaluates to the number of elements in the list
• len(some_string) evaluates to the length of the string (excluding null terminator of course)
• len(some_buffer) evaluates to the number of bytes in the buffer

This is also a handy way to check if lists are empty, since we couldn't do it before, which was apparent after a discussion with @leogr and @jasondellaluce .

Which issue(s) this PR fixes:

Fixes #2127

Special notes for your reviewer:

• The implementation slightly changes the transformer interface which now takes the element flags as well
• Do you see any potential corner cases with plugins?

Does this PR introduce a user-facing change?:

new(libsinsp): add len() filter transformer

[poiana]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: LucaGuerra

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• ~~OWNERS~~ [LucaGuerra]

Approvers can indicate their approval by writing /approve in a comment Approvers can cancel approval by writing /approve cancel in a comment

[github-actions[bot]]

Perf diff from master - unit tests

     5.26%     -1.03%  [.] sinsp_parser::process_event
     4.49%     +0.95%  [.] next
     3.17%     +0.84%  [.] gzfile_read
     3.94%     -0.79%  [.] sinsp_evt::load_params
     6.15%     +0.71%  [.] sinsp::next
     1.70%     -0.62%  [.] libsinsp::sinsp_suppress::process_event
     1.32%     -0.56%  [.] sinsp_parser::event_cleanup
     5.20%     +0.55%  [.] sinsp_evt::get_type
     3.44%     +0.54%  [.] sinsp_thread_manager::get_thread_ref
     2.63%     +0.48%  [.] sinsp_thread_manager::find_thread

Heap diff from master - unit tests

peak heap memory consumption: -861B
peak RSS (including heaptrack overhead): 0B
total memory leaked: 0B

Heap diff from master - scap file

peak heap memory consumption: -32B
peak RSS (including heaptrack overhead): 0B
total memory leaked: 0B

Benchmarks diff from master

Comparing gbench_data.json to /root/actions-runner/_work/libs/libs/build/gbench_data.json
Benchmark                                                         Time             CPU      Time Old      Time New       CPU Old       CPU New
----------------------------------------------------------------------------------------------------------------------------------------------
BM_sinsp_split_mean                                            -0.0285         -0.0285           154           149           153           149
BM_sinsp_split_median                                          -0.0282         -0.0281           153           149           153           149
BM_sinsp_split_stddev                                          +0.9625         +0.9624             1             3             1             3
BM_sinsp_split_cv                                              +1.0201         +1.0199             0             0             0             0
BM_sinsp_concatenate_paths_relative_path_mean                  -0.0018         -0.0018            57            57            57            57
BM_sinsp_concatenate_paths_relative_path_median                -0.0019         -0.0020            57            57            57            57
BM_sinsp_concatenate_paths_relative_path_stddev                -0.2067         -0.2036             0             0             0             0
BM_sinsp_concatenate_paths_relative_path_cv                    -0.2054         -0.2022             0             0             0             0
BM_sinsp_concatenate_paths_empty_path_mean                     +0.0432         +0.0432            24            25            24            25
BM_sinsp_concatenate_paths_empty_path_median                   +0.0405         +0.0404            24            25            24            25
BM_sinsp_concatenate_paths_empty_path_stddev                   -0.2387         -0.2389             0             0             0             0
BM_sinsp_concatenate_paths_empty_path_cv                       -0.2702         -0.2704             0             0             0             0
BM_sinsp_concatenate_paths_absolute_path_mean                  -0.0302         -0.0302            56            55            56            55
BM_sinsp_concatenate_paths_absolute_path_median                -0.0389         -0.0389            56            54            56            54
BM_sinsp_concatenate_paths_absolute_path_stddev                +5.7276         +5.7165             0             1             0             1
BM_sinsp_concatenate_paths_absolute_path_cv                    +5.9373         +5.9259             0             0             0             0
BM_sinsp_split_container_image_mean                            -0.0110         -0.0110           398           393           398           393
BM_sinsp_split_container_image_median                          -0.0125         -0.0124           398           393           398           393
BM_sinsp_split_container_image_stddev                          +0.3031         +0.3030             2             2             2             2
BM_sinsp_split_container_image_cv                              +0.3177         +0.3176             0             0             0             0

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 87.64940% with 31 lines in your changes missing coverage. Please review.

Project coverage is 74.60%. Comparing base (701e991) to head (5263f40). Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
userspace/libsinsp/test/filter_transformer.ut.cpp	87.89%	23 Missing :warning:
userspace/libsinsp/sinsp_filter_transformer.cpp	81.81%	8 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2131      +/-   ##
==========================================
+ Coverage   74.46%   74.60%   +0.13%     
==========================================
  Files         254      254              
  Lines       33340    33458     +118     
  Branches     5724     5720       -4     
==========================================
+ Hits        24827    24961     +134     
+ Misses       8508     8470      -38     
- Partials        5       27      +22     

Flag	Coverage Δ
libsinsp	74.60% <87.64%> (+0.13%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[FedeDP]

/milestone 0.19.0

[LucaGuerra]

Thanks! Addressed comments

[poiana]

LGTM label has been added.

Git tree hash: a224ada55b3058be76302af4ac6ad6a1508ff483