out_splunk: skip the flush if payload is empty

[lecaros]

Addresses #10313

Skips the flush if the payload is empty.
Empty payload can be the result of filtering the data. See #10313 for details.

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

• Example configuration file for the change. See out_splunk tries to send data with empty payload #10313
• Debug log output from testing the change

Relevant message:

[2025/05/08 11:44:10] [ warn] [output:splunk:splunk.1] Content length is 0, skipping flush

/fluent-bit -c drop.yml
Fluent Bit v4.0.2
* Copyright (C) 2015-2025 The Fluent Bit Authors
* Fluent Bit is a CNCF sub-project under the umbrella of Fluentd
* https://fluentbit.io

______ _                  _    ______ _ _             ___  _____ 
|  ___| |                | |   | ___ (_) |           /   ||  _  |
| |_  | |_   _  ___ _ __ | |_  | |_/ /_| |_  __   __/ /| || |/' |
|  _| | | | | |/ _ \ '_ \| __| | ___ \ | __| \ \ / / /_| ||  /| |
| |   | | |_| |  __/ | | | |_  | |_/ / | |_   \ V /\___  |\ |_/ /
\_|   |_|\__,_|\___|_| |_|\__| \____/|_|\__|   \_/     |_(_)___/ 


[2025/05/08 11:44:07] [ info] Configuration:
[2025/05/08 11:44:07] [ info]  flush time     | 1.000000 seconds
[2025/05/08 11:44:07] [ info]  grace          | 5 seconds
[2025/05/08 11:44:07] [ info]  daemon         | 0
[2025/05/08 11:44:07] [ info] ___________
[2025/05/08 11:44:07] [ info]  inputs:
[2025/05/08 11:44:07] [ info]      event_type
[2025/05/08 11:44:07] [ info] ___________
[2025/05/08 11:44:07] [ info]  filters:
[2025/05/08 11:44:07] [ info] ___________
[2025/05/08 11:44:07] [ info]  outputs:
[2025/05/08 11:44:07] [ info]      stdout.0
[2025/05/08 11:44:07] [ info]      splunk.1
[2025/05/08 11:44:07] [ info] ___________
[2025/05/08 11:44:07] [ info]  collectors:
[2025/05/08 11:44:07] [ info] [fluent bit] version=4.0.2, commit=5dd2b0b438, pid=21372
[2025/05/08 11:44:07] [debug] [engine] coroutine stack size: 36864 bytes (36.0K)
[2025/05/08 11:44:07] [ info] [storage] ver=1.1.6, type=memory+filesystem, sync=normal, checksum=off, max_chunks_up=128
[2025/05/08 11:44:07] [ info] [storage] backlog input plugin: storage_backlog.1
[2025/05/08 11:44:07] [ info] [simd    ] disabled
[2025/05/08 11:44:07] [ info] [cmetrics] version=1.0.0
[2025/05/08 11:44:07] [ info] [ctraces ] version=0.6.5
[2025/05/08 11:44:07] [ info] [input:event_type:event_type.0] initializing
[2025/05/08 11:44:07] [ info] [input:event_type:event_type.0] storage_strategy='filesystem' (memory + filesystem)
[2025/05/08 11:44:07] [debug] [input:event_type:event_type.0] [thread init] initialization OK
[2025/05/08 11:44:07] [ info] [input:event_type:event_type.0] thread instance initialized
[2025/05/08 11:44:07] [debug] [event_type:event_type.0] created event channels: read=38 write=39
[2025/05/08 11:44:07] [debug] [processor:metrics_selector:metrics_selector.0] action type EXCLUDE
[2025/05/08 11:44:07] [ info] [input:storage_backlog:storage_backlog.1] initializing
[2025/05/08 11:44:07] [ info] [input:storage_backlog:storage_backlog.1] storage_strategy='memory' (memory only)
[2025/05/08 11:44:07] [debug] [storage_backlog:storage_backlog.1] created event channels: read=42 write=43
[2025/05/08 11:44:07] [ info] [input:storage_backlog:storage_backlog.1] queue memory limit: 95.4M
[2025/05/08 11:44:07] [debug] [stdout:stdout.0] created event channels: read=44 write=45
[2025/05/08 11:44:07] [debug] [processor:metrics_selector:metrics_selector.0] action type EXCLUDE
[2025/05/08 11:44:07] [debug] [splunk:splunk.1] created event channels: read=54 write=55
[2025/05/08 11:44:07] [ info] [output:stdout:stdout.0] worker #0 started
[2025/05/08 11:44:07] [ info] [output:splunk:splunk.1] worker #0 started
[2025/05/08 11:44:07] [debug] [router] match rule event_type.0:stdout.0
[2025/05/08 11:44:07] [ info] [output:splunk:splunk.1] worker #1 started
[2025/05/08 11:44:07] [debug] [router] match rule event_type.0:splunk.1
[2025/05/08 11:44:07] [debug] [router] match rule storage_backlog.1:stdout.0
[2025/05/08 11:44:07] [debug] [router] match rule storage_backlog.1:splunk.1
[2025/05/08 11:44:07] [ info] [sp] stream processor started
[2025/05/08 11:44:09] [debug] [input:event_type:event_type.0] metrics, ret=0
[2025/05/08 11:44:09] [ info] [input:event_type:event_type.0] [OK] collector_time
[2025/05/08 11:44:09] [debug] [input coro] destroy coro_id=0
[2025/05/08 11:44:10] [debug] [task] created task=0x600002d70000 id=0 OK
[2025/05/08 11:44:10] [debug] [output:stdout:stdout.0] task_id=0 assigned to thread #0
[2025/05/08 11:44:10] [debug] [output:splunk:splunk.1] task_id=0 assigned to thread #0
[2025/05/08 11:44:10] [debug] [output:stdout:stdout.0] cmt decode msgpack returned : 1
[2025/05/08 11:44:10] [debug] [out flush] cb_destroy coro_id=0
[2025/05/08 11:44:10] [debug] [upstream] KA connection #93 to localhost:8090 is connected
[2025/05/08 11:44:10] [ warn] [output:splunk:splunk.1] Content length is 0, skipping flush
[2025/05/08 11:44:10] [debug] [upstream] KA connection #93 to localhost:8090 is now available
[2025/05/08 11:44:10] [debug] [out flush] cb_destroy coro_id=0
[2025/05/08 11:44:10] [debug] [task] destroy task=0x600002d70000 (task_id=0)
[2025/05/08 11:44:11] [debug] [input:event_type:event_type.0] metrics, ret=0
[2025/05/08 11:44:11] [ info] [input:event_type:event_type.0] [OK] collector_time
[2025/05/08 11:44:11] [debug] [input coro] destroy coro_id=1
^C[2025/05/08 11:44:12] [engine] caught signal (SIGINT)
[2025/05/08 11:44:12] [ info] [input] pausing storage_backlog.1
[2025/05/08 11:44:12] [ info] [output:stdout:stdout.0] thread worker #0 stopping...
[2025/05/08 11:44:12] [ info] [output:stdout:stdout.0] thread worker #0 stopped
[2025/05/08 11:44:12] [ info] [output:splunk:splunk.1] thread worker #0 stopping...
[2025/05/08 11:44:12] [ info] [output:splunk:splunk.1] thread worker #0 stopped
[2025/05/08 11:44:12] [ info] [output:splunk:splunk.1] thread worker #1 stopping...
[2025/05/08 11:44:12] [ info] [output:splunk:splunk.1] thread worker #1 stopped
[2025/05/08 11:44:12] [debug] [input:event_type:event_type.0] thread exit instance

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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.

[edsiper]

as discussed internally, if a metrics processor can potentially ship a context without metric series, it should be dropped before being ingested into the pipeline.

[lecaros]

moving this to draft while a better solution is designed and implemented