feat: update log level discovery from json to detect nested fields

[ashwanthgoli]

What this PR does / why we need it:

Log level discovery for json lines currently only looks at the top level fields, this helps detect level fields that are nested.
It only does a single pass through the log line for detecting the levels using jsonparser.ObjectEach unlike the existing approach that calls Get for each allowed label.

Note: This approach branches away from getValueUsingJSONParser which was common for both level and generic field detection previously and additionally supported jsonpath expressions.

goos: darwin
goarch: arm64
pkg: github.com/grafana/loki/v3/pkg/distributor
cpu: Apple M1 Pro
                                                                    │ /tmp/before.txt │           /tmp/after.txt            │
                                                                    │     sec/op      │   sec/op     vs base                │
_optParseExtractLogLevelFromLogLineJson/deeply_nested_level_field-8     12721.5n ± 2%   183.0n ± 1%  -98.56% (p=0.000 n=10)
_optParseExtractLogLevelFromLogLineJson/level_field_at_start-8          1139.50n ± 1%   59.45n ± 2%  -94.78% (p=0.000 n=10)
_optParseExtractLogLevelFromLogLineJson/level_field_in_middle-8          1225.5n ± 1%   189.0n ± 6%  -84.58% (p=0.000 n=10)
_optParseExtractLogLevelFromLogLineJson/level_field_at_end-8             1335.5n ± 1%   350.4n ± 3%  -73.76% (p=0.000 n=10)
_optParseExtractLogLevelFromLogLineJson/no_level_field-8                 12.822µ ± 7%   1.098µ ± 3%  -91.44% (p=0.000 n=10)
_optParseExtractLogLevelFromLogLineJson/nested_level_field-8           12597.50n ± 1%   97.86n ± 5%  -99.22% (p=0.000 n=10)

Which issue(s) this PR fixes:
Fixes #15822

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

[shantanualsi]

I like the idea!

You're going depth first in your implementation, I think it would make sense to go by level to reduce the memory usage? eg: {"body": { ...5 level deep json... }, "level": INFO}

I think many log formatters do keep log_level shallow, except for a few customised ones.

[github-actions]

💻 Deploy preview deleted.

[ashwanthgoli]

i think breath first search is going to incur memory-overhead, currently the parser is presenting data as it apprears - we'd encounter children first in the json line before the next sibling

I think many log formatters do keep log_level shallow, except for a few customised ones.

👍 we could default the maxDepth to 1 to avoid any overhead

[shantanualsi]

LGTM