Discover returns bad request error when one doc has a field with 1000000+ Bytes

[jguay]

Kibana version:
7.9.2
Elasticsearch version:
7.9.1
Describe the bug:
This is a result of elastic/elasticsearch#52155 and the fact by default highlighting is enabled on Discover application (disabling it in advanced options)
Steps to reproduce:

1. Create a document with one million bytes, for example :

echo '{ "foo" : "' > payload.json
for i in {1..70000}; do printf 'brown fox test ' >> payload.json; done
echo '"}' >> payload.json
curl -H 'Content-type: application/json' -k -u elastic:changeme -XPOST https://localhost:9200/test/_doc -d @payload.json

2. Create an index pattern for that index
3. Go to Discover and see the error in screenshot below
   Browser network trace shows response from elasticsearch was :

{"statusCode":400,"error":"Bad Request","message":"[status_exception] error while executing search","attributes":{"error":{"type":"status_exception","reason":"error while executing search","caused_by":{"type":"search_phase_execution_exception","reason":"all shards failed","phase":"query","grouped":true,"failed_shards":[{"shard":0,"index":"test","node":"ZsGX5OPCQ8WYjKZsBkZG9g","reason":{"type":"illegal_argument_exception","reason":"The length of [foo] field of [lI-O2XQBFxy84PJko7GD] doc of [test] index has exceeded [1000000] - maximum allowed to be analyzed for highlighting. This maximum can be set by changing the [index.highlight.max_analyzed_offset] index level setting. For large texts, indexing with offsets or term vectors is recommended!"}}],"caused_by":{"type":"illegal_argument_exception","reason":"The length of [foo] field of [lI-O2XQBFxy84PJko7GD] doc of [test] index has exceeded [1000000] - maximum allowed to be analyzed for highlighting. This maximum can be set by changing the [index.highlight.max_analyzed_offset] index level setting. For large texts, indexing with offsets or term vectors is recommended!","caused_by":{"type":"illegal_argument_exception","reason":"The length of [foo] field of [lI-O2XQBFxy84PJko7GD] doc of [test] index has exceeded [1000000] - maximum allowed to be analyzed for highlighting. This maximum can be set by changing the [index.highlight.max_analyzed_offset] index level setting. For large texts, indexing with offsets or term vectors is recommended!"}}}}}}

Stack trace from screenshot :

search_phase_execution_exception
all shards failed

Error: Bad Request
    at Fetch._callee3$ (https://localhost:5601/33984/bundles/core/core.entry.js:34:109213)
    at l (https://localhost:5601/33984/bundles/kbn-ui-shared-deps/kbn-ui-shared-deps.js:368:155323)
    at Generator._invoke (https://localhost:5601/33984/bundles/kbn-ui-shared-deps/kbn-ui-shared-deps.js:368:155076)
    at Generator.forEach.e.<computed> [as next] (https://localhost:5601/33984/bundles/kbn-ui-shared-deps/kbn-ui-shared-deps.js:368:155680)
    at fetch_asyncGeneratorStep (https://localhost:5601/33984/bundles/core/core.entry.js:34:102354)
    at _next (https://localhost:5601/33984/bundles/core/core.entry.js:34:102670)

Expected behavior:
More meaningful error - or no error

Screenshots (if relevant):

Any additional context:
I am mostly opening this to ensure this is tracked along with elastic/elasticsearch#52155, the latest proposition of a fix to return partial highlighting instead of error at elasticsearch level should require no change on kibana (the previous proposition with some ignore_above if it is what is implemented would required kibana code change)

[elasticmachine]

Pinging @elastic/kibana-app-arch (Team:AppArch)

[kertal]

this may have improved with recent error message alignment (because there's a clear message in the return: doc of [test] index has exceeded [1000000] - maximum allowed to be analyzed for highlighting. This maximum can be set by changing the [index.highlight.max_analyzed_offset] index level setting...), but should be checked (FYI: @lizozom )