chore: RFC for Data Volume Insights (vectordotdev#17322)

[Rendered](https://github.com/vectordotdev/vector/blob/stephen/data_volume_rfc/rfcs/2023-05-03-data-volume-metrics.md)

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Signed-off-by: Stephen Wakely <fungus.humungus@gmail.com>

rfcs/2023-05-03-data-volume-metrics.md

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+# RFC 2023-05-02 - Data Volume Insights metrics
+
+Vector needs to be able to emit accurate metrics that can be usefully queried
+to give users insights into the volume of data moving through the system.
+
+## Scope
+
+### In scope
+
+- All volume event metrics within Vector need to emit the estimated JSON size of the
+  event. With a consistent method for determining the size it will be easier to accurately
+  compare data in vs data out.
+  - `component_received_event_bytes_total`
+  - `component_sent_event_bytes_total`
+  - `component_received_event_total`
+  - `component_sent_event_total`
+- The metrics sent by each sink needs to be tagged with the source id of the
+  event so the route an event takes through Vector can be queried.
+- Each event needs to be labelled with a `service`. This is a new concept
+  within Vector and represents the application that generated the log,
+  metric or trace.
+- The service tag and source tag in the metrics needs to be opt in so customers
+  that don't need the increased cardinality are unaffected.
+
+### Out of scope
+
+- Separate metrics, `component_sent_bytes_total`  and `component_received_bytes_total`
+  that indicate network bytes sent by Vector are not considered here.
+
+## Pain
+
+Currently it is difficult to accurately gauge the volume of data that is moving
+through Vector. It is difficult to query where data being sent out has come
+from.
+
+## Proposal
+
+### User Experience
+
+Global config options will be provided to indicate that the `service` tag and the
+`source` tag should be sent. For example:
+
+```yaml
+telemetry:
+  tags:
+    service: true
+    source_id: true
+```
+
+This will cause Vector to emit a metric like (note the last two tags):
+
+```prometheus
+vector_component_sent_event_bytes_total{component_id="out",component_kind="sink",component_name="out",component_type="console"
+                                        ,host="machine",service="potato",source_id="stdin"} 123
+```
+
+The default will be to not emit these tags.
+
+### Implementation
+
+#### Metric tags
+
+**service** - to attach the service, we need to add a new meaning to Vector -
+              `service`. Any sources that receive data that could potentially
+              be considered a service will need to indicate which field means
+              `service`. This work has largely already been done with the
+              LogNamespacing work, so it will be trivial to add this new field.
+              Not all sources will be able to specify a specific field to
+              indicate the `service`. In time it will be possible for this to
+              be accomplished through `VRL`.
+
+**source_id** - A new field will be added to the [Event metadata][event_metadata] -
+                `Arc<OutputId>` that will indicate the source of the event.
+                `OutputId` will need to be serializable so it can be stored in
+                the disk buffer. Since this field is just an identifier, it can
+                still be used even if the source no longer exists when the event
+                is consumed by a sink.
+
+We will need to do an audit of all components to ensure the
+bytes emitted for the `component_received_event_bytes_total` and
+`component_sent_event_bytes_total` metrics are the estimated JSON size of the
+event.
+
+These tags will be given the name that was configured in [User Experience]
+(#user-experience).
+
+Transforms `reduce` and `aggregate` combine multiple events together. In this
+case the `source` and `service` of the first event will be taken.
+
+If there is no `source` a source of `-` will be emitted. The only way this can
+happen is if the event was created by the `lua` transform.
+
+If there is no `service` available, a service of `-` will be emitted.
+
+Emitting a `-` rather than not emitting anything at all makes it clear that
+there was no value rather than it just having been forgotten and ensures it
+is clear that the metric represents no `service` or `source` rather than the
+aggregate value across all  services.
+
+The [Component Spec][component_spec] will need updating to indicate these tags
+will need including.
+
+**Performance** - There is going to be a performance hit when emitting these metrics.
+Currently for each batch a simple event is emitted containing the count and size
+of the entire batch. With this change it will be necessary to scan the entire
+batch to obtain the count of source, service combinations of events before emitting
+the counts. This will involve additional allocations to maintain the counts as well
+as the O(1) scan.
+
+#### `component_received_event_bytes_total`
+
+This metric is emitted by the framework [here][source_sender], so it looks like
+the only change needed is to add the service tag.
+
+#### `component_sent_event_bytes_total`
+
+For stream based sinks this will typically be the byte value returned by
+`DriverResponse::events_sent`.
+
+Despite being in the [Component Spec][component_spec], not all sinks currently
+conform to this.
+
+As an example, from a cursory glance over a couple of sinks:
+
+The Amqp sink currently emits this value as the length of the binary
+data that is sent. By the time the data has reached the code where the
+`component_sent_event_bytes_total` event is emitted, that event has been
+encoded and the actual estimated JSON size has been lost. The sink will need
+to be updated so that when the event is encoded, the encoded event together
+with the pre-encoded JSON bytesize will be sent to the service where the event
+is emitted.
+
+The Kafka sink also currently sends the binary size, but it looks like the
+estimated JSON bytesize is easily accessible at the point of emitting, so would
+not need too much of a change.
+
+To ensure that the correct metric is sent in a type-safe manner, we will wrap
+the estimated JSON size in a newtype:
+
+```rust
+pub struct JsonSize(usize);
+```
+
+The `EventsSent` metric will only accept this type.
+
+### Registered metrics
+
+It is currently not possible to have dynamic tags with preregistered metrics.
+
+Preregistering these metrics are essential to ensure that they don't expire.
+
+The current mechanism to expire metrics is to check if a handle to the given
+metric is being held. If it isn't, and nothing has updated that metric in
+the last cycle - the metric is dropped. If a metric is dropped, the next time
+that event is emitted with those tags, the count starts at zero again.
+
+We will need to introduce a registered event caching layer that will register
+and cache new events keyed on the tags that are sent to it.
+
+Currently a registered metrics is stored in a `Registered<EventSent>`.
+
+We will need a new struct that can wrap this that will be generic over a tuple of
+the tags for each event and the event - eg. `Cached<(String, String), EventSent>`.
+This struct will maintain a BTreeMap of tags -> `Registered`. Since this will
+need to be shared across threads, the cache will need to be stored in an `RwLock`.
+
+In pseudo rust:
+
+```rust
+struct Cached<Tags, Event> {
+  cache: Arc<RwLock<BTreemap<Tags, Registered<Event>>>,
+  register: Fn(Tags) -> Registered<Event>,
+}
+
+impl<Tags, Event> Cached<Tags, Event> {
+  fn emit(&mut self, tags: Tags, value: Event) -> {
+    if Some(event) = self.cache.get(tags) {
+      event.emit(value);
+    } else {
+      let event = self.register(tags);
+      event.emit(value);
+      self.cache.insert(tags, event);
+    }
+  }
+}
+```
+
+## Rationale
+
+The ability to visualize data flowing through Vector will allow users to ascertain
+the effectiveness of the current use of Vector. This will enable users to
+optimise their configurations to make the best use of Vector's features.
+
+## Drawbacks
+
+The additional tags being added to the metrics will increase the cardinality of
+those metrics if they are enabled.
+
+## Prior Art
+
+
+## Alternatives
+
+We could use an alternative metric instead of estimated JSON size.
+
+- *Network bytes* This provides a more accurate picture of the actual data being received
+  and sent by Vector, but will regularly produce different sizes for an incoming event
+  to an outgoing event.
+- *In memory size* The size of the event as held in memory. This may be more accurate in
+  determining the amount of memory Vector will be utilizing at any time, will often be
+  less accurate compared to the data being sent and received which is often JSON.
+
+## Outstanding Questions
+
+## Plan Of Attack
+
+Incremental steps to execute this change. These will be converted to issues after the RFC is approved:
+
+- [ ] Add the `source` field to the Event metadata to indicate the source the event has come from.
+- [ ] Update the Volume event metrics to take a `JsonSize` value. Use the compiler to ensure all metrics
+      emitted use this. The `EstimatedJsonEncodedSizeOf` trait will be updated return a `JsonSize`.
+- [ ] Add the Service meaning. Update any sources that potentially create a service to point the meaning
+      to the relevant field.
+- [ ] Introduce an event caching layer that caches registered events based on the tags sent to it.
+- [ ] Update the emitted events to accept the new tags - taking the `telemetry` configuration options
+      into account.
+- [ ] There is going to be a hit on performance with these changes. Add benchmarking to help us understand
+      how much the impact will be.
+
+## Future Improvements
+
+- Logs emitted by Vector should also be tagged with `source_id` and `service`.
+- This rfc proposes storing the source and service as strings. This incurs a cost since scanning each
+  event to get the counts of events by source and service will involve multiple string comparisons. A
+  future optimization could be to hash the combination of these values at the source into a single
+  integer.
+
+[component_spec]: https://github.com/vectordotdev/vector/blob/master/docs/specs/component.md#componenteventssent
+[source_sender]: https://github.com/vectordotdev/vector/blob/master/src/source_sender/mod.rs#L265-L268
+[event_metadata]: https://github.com/vectordotdev/vector/blob/master/lib/vector-core/src/event/metadata.rs#L20-L38