Make journald emit map[string]interface{}

[camdencheek]

Description of Changes

It turns out our entry.Field doesn't handle iterating through map[string]string well. We ran into this because the journald plugin emits map[string]string. This PR changes the journald input to be more in line with the other plugins so that it emits a map[string]interface{}.

This seems to be part of a larger recurring issue with the types that are allowed in our pipeline. We should really consider locking down the types that are allowed into the record in the first place. It's impossible to handle every type, and if we don't have any standards, it's unclear what is and is not expected to be handled by each of the downstream plugins.

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

[codecov]

Codecov Report

Merging #38 into master will decrease coverage by 0.18%.
The diff coverage is 55.56%.

@@            Coverage Diff             @@
##           master      #38      +/-   ##
==========================================
- Coverage   75.44%   75.27%   -0.18%     
==========================================
  Files          61       61              
  Lines        3718     3724       +6     
==========================================
- Hits         2805     2803       -2     
- Misses        690      696       +6     
- Partials      223      225       +2     

Impacted Files	Coverage Δ
plugin/builtin/input/journald.go	55.10% <55.56%> (-1.42%)	⬇️
plugin/builtin/output/file.go	51.22% <0.00%> (-9.76%)	⬇️

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[djaglowski]

Log Files	Logs / Second	CPU Average (%)	Memory Average (MB)
1	1000	3.6379902	28.063173
1	5000	6.4311776	36.749462
1	10000	9.431189	43.473732
1	50000	35.484413	118.47549
1	100000	62.56923	192.38362
10	100	4.1379976	28.379715
10	500	6.9311438	32.389008
10	1000	11.086395	45.661232
10	5000	43.345715	86.875946
10	10000	79.983376	199.24582