fix: merge per-tenant overrides with defaults for unset fields

[electron0zero]

What this PR does:

Fixes a bug where per-tenant and wildcard overrides that only set a few fields cause all other fields to resolve to Go zero values instead of falling back to static defaults.

Bug: When a tenant has an override entry that sets only a subset of fields (e.g., just max_cardinality_per_label: 100), all other fields resolve to Go zero values instead of the static defaults. This is because the override struct is returned as-is without merging in the defaults for unset fields.

Fix: Each override entry is now field-level merged with static defaults at config load time using reflection. Unset fields fall back to defaults instead of being zero values. Merged views are pre-computed at load/reload - zero cost per request.

Fields where zero is semantically meaningful (e.g., 0 = "no limit", false = "disabled") are migrated to pointer types so the merge can distinguish "not set" (nil) from "explicitly zero" (*0).

This is needed because the merge uses "is this field zero?" to decide whether a field was set in the override file.

For most fields this works fine - a zero value means "not set". But some fields use zero as a real value: max_bytes_per_trace: 0 means "no limit", span_multiplier_key: "" means "disable multiplier".

Without pointers, the merge would treat these as "not set" and overwrite them with the default, making it impossible for operators to explicitly set these values.

Pointer types solve this because we can check nil and assign it to "not set, fall back to default", non-nil zero = "explicitly set to this value".

Behavior before this PR:

• Lookup returns the first matching override struct whole (per-tenant OR wildcard OR defaults)
• If a per-tenant override exists, it is returned as-is - fields not set in the override file resolve to Go zero values, not the static defaults
• If a wildcard tenant override exists, it is returned as-is - fields not set in the override file resolve to Go zero values, not the static defaults
• if both per tenant and wildcard tenant override doesn't exist, returns default

Behavior after this PR:

• Lookup order is unchanged, it's still per-tenant OR wildcard OR defaults
• The matched override entry is now field-level merged with static defaults - fields not set in the override file correctly fall back to the static defaults

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[electron0zero]

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[electron0zero]

@codex review again

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