filter_nest: define a new var to prevent pointer arithmetic

[nokute78]

#8137

An arg of FLB_LOG_EVENT_STRING_LENGTH_VALUE is a flb_log_event_encoder_size_t.
flb_log_event_encoder_size_t is size_t* on Windows.
It can cause pointer arithmetic and the output can be larger value.
e.g. FLB_LOG_EVENT_STRING_LENGTH_VALUE(10+3) can be 10 + sizeof(size_t*) * 3 = 34.

We use 'new_size' to prevent pointer arithmetic.

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

• [N/A] Attached Valgrind output that shows no leaks or memory corruption was found

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• [N/A] Run local packaging test showing all targets (including any new ones) build.
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Documentation

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Backporting

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

Configuration

[SERVICE]
    Log_Level debug

[INPUT]
    Name dummy
    Dummy {"map_data":{"key":"value", "id":123}}
    samples 1

[FILTER]
    Name nest
    Match *
    Operation lift
    Nested_under map_data
    Add_prefix _map_data.

[OUTPUT]
    name stdout
    match *

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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.

[janlonsetteig]

Hopefully there will be a review of this fix soon so that we can get it released

[patrick-stephens]

I've added the labels to attempt to get you a Windows container @nokute78