Operation Insights: Metadata Tags for operations

It would be great if Hive reports were taggable and filterable by custom user-defined tags. Use cases for tags could include

- Region (e.g. europe-west, asia-pacific, north-america) for examining perfomance per Region
- Backend-Revision - examining revision-specific performance
- Tenant-name - enabling multitenancy analysis

Essentially, tags complement `ClientInfo` to provide additional metadata on the request. An operation could have one or multiple tags, resulting in the following changes to the Hive Reporting Spec:

```diff
export interface RequestOperation {
timestamp: number
operationMapKey: string
execution: Execution
metadata?: Metadata
}

export interface Metadata {
client?: Client
+ tag?: string // OR
+ tags?: string[]
}
```

In the first iteration, the insights UI could be adjusted to have an additional filter like this:

If you agree on this feature being useful, there's several points to discuss:

- One vs Multiple tags - (utility vs DB index performance)
- Necessity of additional ClickHouse materialized views for Tags (complementing client, operation views)
- Default tag if none was provided (better DB performance), similar to client -> "unknown"

Looking forward to your opinion on this.

Happy to draft an initial PR once we are aligned on the open points.