[8.19](backport #45076) [OTel] Convert []any-nested mapstr.M

[mergify]

This adds the reflective capture of mapstr.M in addition to map[string]any for []any.

I noticed, at runtime, the cat_shards metricset produces mapstr.M in a nested array even though the autoops_es code does not produce it that way, libbeat does:

beats/libbeat/common/event.go

Lines 240 to 246 in 0dd36b9

	case reflect.Map, reflect.Struct:
	var m mapstr.M
	err := marshalUnmarshal(value, &m)
	if err != nil {
	return m, []error{fmt.Errorf("key=%v: error converting %T to mapstr.M: %w", joinKeys(keys...), value, err)}
	}
	return m, nil

Proposed commit message

Convert inner slice/array mapstr.M to map[string]any.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Disruptive User Impact

None. This improves compatibility.

Author's Checklist

• Validate tests.

How to test this PR locally

Run the Elastic Agent with autoops_es

Related issues

• Relates [OTel Mapper] Fails to convert nested arrays and slices to []any #45007

  ---

  This is an automatic backport of pull request [OTel] Convert []any-nested mapstr.M #45076 done by Mergify.

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[pickypg]

Conflict requires #45080