WIP: feat(discover): Query specific datasets in events-stats

[narsaynorath]

I tried to update the events-stats endpoint to query for the specific datasets but I'm getting this error:

...
  File "/Users/nsaynorath/workspace/sentry/src/sentry/search/events/datasets/discover.py", line 1472, in _resolve_web_vital_function
    raise InvalidSearchQuery("count_web_vitals only supports measurements")
sentry.exceptions.InvalidSearchQuery: count_web_vitals only supports measurements

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/nsaynorath/workspace/sentry/src/sentry/snuba/metrics/extraction.py", line 702, in _extract_aggregate_components
    function, args, _ = _parse_function(aggregate)
                        ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/nsaynorath/workspace/sentry/src/sentry/snuba/metrics/extraction.py", line 545, in _parse_function
    raise InvalidSearchQuery(f"Invalid characters in field {aggregate}")
sentry.exceptions.InvalidSearchQuery: Invalid characters in field customtag2
========================================= short test summary info =========================================
FAILED tests/snuba/api/endpoints/test_organization_events_stats_mep.py::OrganizationEventsStatsMetricsEnhancedPerformanceEndpointTestWithOnDemandWidgets::test_top_events_with_transaction_on_demand_passing_widget_id_unsaved_transaction_only - AssertionError: assert None == 101
================================= 1 failed, 82 passed in 67.56s (0:01:07) =================================

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 78.09%. Comparing base (7774556) to head (78ff982).
Report is 1018 commits behind head on master.

Files	Patch %	Lines
.../sentry/api/endpoints/organization_events_stats.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #75118      +/-   ##
==========================================
- Coverage   79.91%   78.09%   -1.82%     
==========================================
  Files        6926     6755     -171     
  Lines      311015   301456    -9559     
  Branches    51861    51861              
==========================================
- Hits       248562   235437   -13125     
+ Misses      62030    59689    -2341     
- Partials      423     6330    +5907     

Files	Coverage Δ
.../sentry/api/endpoints/organization_events_stats.py	91.48% <0.00%> (-2.27%)	⬇️

... and 1764 files with indirect coverage changes