Add event Initialized() to Initializable

[kevupton]

Copied over from OpenZeppelin/openzeppelin-contracts-upgradeable#148

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Added an event for an initializable contract. To allow for easy identification of when a contract has been initialized.

Suggestion for tracking when a contract has been initialized.

This is helpful in cases like when using TheGraph when we want to do some action on initialize. Such as reading the data and saving it. As some of the data may not be available on construct for example.

Could potentially use event Initialized(address sender); with the sender as well. However, thought it might be better to just keep it simple.

Not sure if there are any negative side effects of this.

PR Checklist

• Tests
• Documentation
• Changelog entry

[frangio]

Thanks for the suggestion @kevupton. This may make sense, I'm just not fully convinced yet.

This is helpful in cases like when using TheGraph when we want to do some action on initialize. Such as reading the data and saving it. As some of the data may not be available on construct for example.

Generally the initializer is called during construction of the proxy, or at least in the same transaction, so I'm not fully convinced by this motivation. @Amxx What are your thoughts on this?

One concern that I have is how this will interact with #2901: we will likely have a reinitializer(n) modifier, so we may want this event to reflect that with an extra uint argument.

[Amxx]

Bumping up this now that we have the reinitializer mechanism.

[Amxx]

Implemented in #3294