Add primary language P3A metric

[DJAndries]

Add a monthly P3A metric for the user's primary language:

What is your primary language?
Brave.Core.PrimaryLang

List of answers defined in components/misc_metrics/language_metrics.cc

Only available via STAR/Constellation.

cc @AndyAnds265

[GeetaSarvadnya]

Verification PASSED on

Brave | 1.64.90 Chromium: 122.0.6261.94 (Official Build) beta (64-bit)
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Revision | 726c6970a1e9c057c69d80e03ad95a7c561fbfe7
OS | Windows 10 Version 22H2 (Build 19045.4046)

• Verified the tets plan from Add primary language metric brave-core#21355
• Confirmed that the metrics Brave.Core.PrimaryLang is shown under logs_constellation_prep_slow and constellation_logs_slow blocks
• The metrics index value isn't matching with the browser language hence logged a follow up issue Follow up of #34853 - The metric index value isn't matches the primary language set in the browser #36529

Example	Example

[hffvld]

Verified on Pixel 7 using version(s):

Device/OS: Pixel 7 / panther_beta-user 14 AP21.240119.009 release-keys
Brave build: 1.64.95
Chromium: 122.0.6261.94 (Official Build) beta (64-bit)

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STEPS:

1. Follow the steps from Add primary language metric brave-core#21355 (comment)
2. Verify

ACTUAL RESULTS:

• Verified that the metrics Brave.Core.PrimaryLang is shown under logs_constellation_prep_slow block

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English	Russian	Spanish	Japanise	Hebrew