filter fields and meta

[sgratzl]

closes #189 subset of fields and closes #197 filtering covidcast meta

new parameter: fields allowing to specify a subset of the fields to be returned. This is mostly for the website to limit the transferred data

[krivard]

Rebase this against main, if you would? There's a schema update that needs to get propagated into integrations/.../test_covidcast.py:test_fields:

Warning: repos.delphi.delphi-epidata.integrations.server.test_covidcast missing attribute __test_target__. Coverage will not be tracked.
test_fields (repos.delphi.delphi-epidata.integrations.server.test_covidcast.CovidcastTests)
Test to limit fields field ... ERROR
test_location_timeline (repos.delphi.delphi-epidata.integrations.server.test_covidcast.CovidcastTests)
Select a timeline for a particular location. ... ok
test_location_wildcard (repos.delphi.delphi-epidata.integrations.server.test_covidcast.CovidcastTests)
Select all locations with a wildcard query. ... ok
test_nullable_columns (repos.delphi.delphi-epidata.integrations.server.test_covidcast.CovidcastTests)
Missing values should be surfaced as null. ... ok
test_round_trip (repos.delphi.delphi-epidata.integrations.server.test_covidcast.CovidcastTests)
Make a simple round-trip with some sample data. ... ok
test_temporal_partitioning (repos.delphi.delphi-epidata.integrations.server.test_covidcast.CovidcastTests)
Request a signal that's available at multiple temporal resolutions. ... ok
test_unique_key_constraint (repos.delphi.delphi-epidata.integrations.server.test_covidcast.CovidcastTests)
Don't allow a row with a key collision to be inserted. ... ok

======================================================================
ERROR: test_fields (repos.delphi.delphi-epidata.integrations.server.test_covidcast.CovidcastTests)
Test to limit fields field
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/src/app/repos/delphi/delphi-epidata/integrations/server/test_covidcast.py", line 85, in test_fields
    self.cur.execute('''
  File "/usr/local/lib/python3.8/site-packages/mysql/connector/cursor.py", line 551, in execute
    self._handle_result(self._connection.cmd_query(stmt))
  File "/usr/local/lib/python3.8/site-packages/mysql/connector/connection.py", line 490, in cmd_query
    result = self._handle_result(self._send_cmd(ServerCmd.QUERY, query))
  File "/usr/local/lib/python3.8/site-packages/mysql/connector/connection.py", line 395, in _handle_result
    raise errors.get_exception(packet)
mysql.connector.errors.DataError: 1136 (21S01): Column count doesn't match value count at row 1

----------------------------------------------------------------------
Ran 7 tests in 0.201s

FAILED (errors=1)

[sgratzl]

main merged and CI is successful again https://github.com/sgratzl/delphi-epidata/runs/1080393029?check_suite_focus=true