GCP pubsub sink should set the message_id depending on settings

[pocesar]

When emitting the events to pubsub for example, would be nice to have the message_id to be set to arbitrary values defined in the TOML.
For example, I'd want the block_hash to be set as the message_id when receiving block events, transaction_hash when receiving transaction events, etc

And also would be nice to be able to set ordering_key as well, to be able to order blocks, transactions either using a static ordering_key or getter from the field (like in the message_id case)

[falcucci]

@pocesar checking about the possibility of this feature,

seems like the publisher has no such control to send a custom message_id based on such rules, once this attr is reserved to be already setted by the server only.

/// ID of this message, assigned by the server when the message is published.
/// Guaranteed to be unique within the topic. This value may be read by a
/// subscriber that receives a `PubsubMessage` via a `Pull` call or a push
/// delivery. It must not be populated by the publisher in a `Publish` call.
#[prost(string, tag = "3")]
pub message_id: ::prost::alloc::string::String,

As an alternative we could maybe proceed with this, adding a custom attribute, meaning that we could send always as possible the block_hash and transaction_hash respectively as field/value to the message.

In that case we should decide about:

• have both unique custom attributes;
• unique attr with dynamic value;

which option would fit your use case better?

cc @scarmuega @paulobressan

[pocesar]

@falcucci makes sense, but the missing ordering key is actually giving issues when publishing, we need to make sure that the order of blocks are linear