doc: minor updates to consumption-metrics document (#7153)

## Problem
Proposed minor changes to the `consumption_metrics` document.

## Summary of changes
- Fixed minor typos in the document.
- Minor formatting in the description of metrics `timeline_logical_size`
and `synthetic_storage_size`. Makes this consistent as with description
  of other metrics in the document.

## Checklist before requesting a review

- [x] I have performed a self-review of my code.
- [ ] If it is a core feature, I have added thorough tests.
- [ ] Do we need to implement analytics? if so did you add the relevant
metrics to the dashboard?
- [ ] If this PR requires public announcement, mark it with
/release-notes label and add several sentences in this section.

## Checklist before merging

- [ ] Do not forget to reformat commit message to not include the above
checklist

Co-authored-by: Mikhail Kot <mikhail@neon.tech>

docs/consumption_metrics.md

@@ -13,7 +13,7 @@ For design details see [the RFC](./rfcs/021-metering.md) and [the discussion on
 batch format is
 ```json
 
-{ "events" : [metric1, metric2, ...]]}
+{ "events" : [metric1, metric2, ...] }
 
 ```
 See metric format examples below.
@@ -49,11 +49,13 @@ Size of the remote storage (S3) directory.
 This is an absolute, per-tenant metric.
 
 - `timeline_logical_size`
-Logical size of the data in the timeline
+
+Logical size of the data in the timeline.
 This is an absolute, per-timeline metric.
 
 - `synthetic_storage_size`
-Size of all tenant's branches including WAL
+
+Size of all tenant's branches including WAL.
 This is the same metric that `tenant/{tenant_id}/size` endpoint returns.
 This is an absolute, per-tenant metric.
 
@@ -106,10 +108,10 @@ This is an incremental, per-endpoint metric.
 ```
 
 The metric is incremental, so the value is the difference between the current and the previous value.
-If there is no previous value, the value, the value is the current value and the `start_time` equals `stop_time`.
+If there is no previous value, the value is the current value and the `start_time` equals `stop_time`.
 
 ### TODO
 
 - [ ] Handle errors better: currently if one tenant fails to gather metrics, the whole iteration fails and metrics are not sent for any tenant.
 - [ ] Add retries
-- [ ] Tune the interval
+- [ ] Tune the interval