Add an extendable metadata field to LoggableElement.
Add an extendable metadata field to LoggableElement.

This is the first step of allowing VE metadata. Later CLs will hook this up.

This required switching the logging config proto from proto3 to proto2 because only proto2 supports extensions.

GITHUB_BREAKING_CHANGES=none

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=316482021

yext/closure-templates,google/closure-templates,yext/closure-templates,google/closure-templates,google/closure-templates,yext/closure-templates,google/closure-templates,google/closure-templates,yext/closure-templates