Resolve journald argument re-use

[jsirianni]

Description of Changes

Now that journald polls for logs (instead of a single long running process), the way arguments are handled needs to be modified. Right now, every poll cycle will append --after-cursor to the args []string value. These parameters are preserved, resulting in many --after-cursor arguments being passed to journald.

Please check that the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• Add a changelog entry (for non-trivial bug fixes / features)
• CI passes

[djaglowski]

Log Files	Logs / Second	CPU Avg (%)	CPU Avg Δ (%)	Memory Avg (MB)	Memory Avg Δ (MB)
1	1000	1.5689743	+0.08622837	129.73761	-0.45742798
1	5000	5.2414584	+0.034559727	137.77344	-0.5580597
1	10000	11.241461	+0.86194515	145.16986	-0.25726318
1	50000	51.000954	+1.6067657	175.78569	+2.4690094
1	100000	108.9007	+8.437637	248.86772	+8.924301
10	100	1.9482888	-0.017226577	133.16595	-0.15840149
10	500	5.982695	+0.051532745	139.9988	+1.3216705
10	1000	11.638024	+0.22399235	144.53233	-2.370407
10	5000	55.61294	+2.4057465	177.94518	-2.0328674
10	10000	114.47258	+4.2478104	238.57584	+10.594696

[codecov]

Codecov Report

Merging #389 (6e54576) into master (74fc7a1) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #389      +/-   ##
==========================================
- Coverage   73.25%   73.23%   -0.02%     
==========================================
  Files         124      124              
  Lines        7994     7995       +1     
==========================================
- Hits         5856     5855       -1     
- Misses       1640     1646       +6     
+ Partials      498      494       -4     

Impacted Files	Coverage Δ
operator/builtin/input/journald/journald.go	57.48% <0.00%> (-0.46%)	⬇️
operator/builtin/output/forward/forward.go	60.49% <0.00%> (-1.23%)	⬇️

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[djaglowski]

A couple very minor optional points. Looks good though.

[djaglowski]

Log Files	Logs / Second	CPU Avg (%)	CPU Avg Δ (%)	Memory Avg (MB)	Memory Avg Δ (MB)
1	1000	1.5000101	+0.017264247	130.47467	+0.27963257
1	5000	5.6209	+0.41400146	135.13133	-3.2001648
1	10000	10.32766	-0.05185604	144.79553	-0.6315918
1	50000	50.725185	+1.3309975	178.05402	+4.737335
1	100000	97.62879	-2.8342743	229.12944	-10.81398
10	100	1.9828467	+0.017331362	132.23195	-1.0924072
10	500	5.9828043	+0.05164194	137.92268	-0.7544403
10	1000	11.98264	+0.5686083	147.7733	+0.8705597
10	5000	53.638985	+0.4317894	179.94194	-0.036102295
10	10000	110.33655	+0.11177826	236.23155	+8.250412