add Request Body Size Limiter

[jranson]

This patch fixes #808 and #806 by centralizing reading of the request body to a middleware handler that also implements the size limiter using an io.LimitReader as part of the centralized read. The default max request body size is 10MB but can be udpated in the config yaml. This patch also addresses POST-related defects in the ALB due to incompatibilities between the various non-centralized, slightly different DRY body reading implementations that are now refactored.

[coveralls]

Pull Request Test Coverage Report for Build 15811013793

Details

• 86 of 175 (49.14%) changed or added relevant lines in 19 files are covered.
• 5 unchanged lines in 5 files lost coverage.
• Overall coverage decreased (-0.2%) to 76.335%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/daemon/setup/setup.go	0	1	0.0%
pkg/proxy/engines/proxy_request.go	21	23	91.3%
pkg/proxy/handlers/influxdb/influxdb.go	0	2	0.0%
pkg/backends/alb/mech/tsm/time_series_merge.go	6	9	66.67%
pkg/frontend/options/options.go	14	17	82.35%
pkg/proxy/handlers/clickhouse/clickhouse.go	0	3	0.0%
pkg/proxy/handlers/prometheus/prometheus.go	0	3	0.0%
pkg/proxy/handlers/rpc/rpc.go	0	3	0.0%
pkg/proxy/handlers/trickster/failures/failures.go	0	7	0.0%
pkg/proxy/response/merge/merge.go	0	9	0.0%

Files with Coverage Reduction	New Missed Lines	%
pkg/frontend/options/options.go	1	67.57%
pkg/proxy/handlers/clickhouse/clickhouse.go	1	0.0%
pkg/proxy/handlers/influxdb/influxdb.go	1	0.0%
pkg/proxy/handlers/prometheus/prometheus.go	1	0.0%
pkg/proxy/handlers/rpc/rpc.go	1	0.0%

Totals
Change from base Build 15785210143:	-0.2%
Covered Lines:	15315
Relevant Lines:	20063

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💛 - Coveralls