fix(ingest/grafana): filter out invalid input field workunits

[anshbansal]

• Running grafana connector against an instance connected to prometheus brought up many issues. This adds a auto work unit so that we do not start hitting server with invalid inputs to charts. The schema field urn being created in that case is wrong. This will require further work by someone more familiar with grafana connector to improve it to make it work with prometheus datasource.
• Small fixes in grafana connector that caused the whole ingestion to fail

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 8 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
.../api/auto_work_units/auto_validate_input_fields.py	80.00%	7 Missing ⚠️
...ion/src/datahub/ingestion/source/grafana/models.py	66.66%	1 Missing ⚠️

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