feat(metrics): Add metadata for buckets

[Dav1dde]

Adds BucketMetadata to the metrics Bucket. The metadata can be used to track meta information about the bucket, like amount of aggregations (total merges).

The metadata currently only tracks the amount of "merges".

Bucket metadata is a purely Relay internal detail, clients are not expected to send it and Relay will ignore and reset the metadata if it is not coming from a trusted/internal Relay. This makes it possible to to implement the received timestamp for metrics while at the same time forwarding the metadata correctly from PoPs to Processing Relays without any additional configurations.

This PR contains:

• Implementation of "merges" metadata
• Extraction of Relay Id from Envelope requests
• Resetting the metadata from unknown/non-internal sources

The metadata is currently unused, I will add all missing integration tests once it is possible to properly assert the metadata in an integration test. I have manually verified that the aggregation of metadata is working.

Change is smaller than it looks, most of it is just test snapshots.

[jjbayer]

Looks good! I'm unsure about the .await in from_request_parts.

What happens when a bucket goes into the pre-aggregator because a project state has not been loaded? Will the accounting be correct?