Fix panic in cassandra storage while registering metrics

[adityacs]

What this PR does / why we need it:
Fixes panic in cassandra storage while registering metrics.

Cassandra storage panics on registering prometheus metrics as there is a mismatch in labels. Loki was missing component="table-manager-store in Desc{fqName: "gocql_connection_pool_hosts", help: "Current number of hosts in the connection pool.", constLabels: {client="table-manager"}, variableLabels: []}

panic: a previously registered descriptor with the same fully-qualified name as Desc{fqName: "gocql_connection_pool_hosts", help: "Current number of hosts in the connection pool.", constLabels: {client="index-read",component="index-store-2020-06-11"}, variableLabels: []} has different label names or a different help string

goroutine 1 [running]:
github.com/prometheus/client_golang/prometheus.(*wrappingRegisterer).MustRegister(0xc00097d440, 0xc00096cc40, 0x1, 0x1)
	/pkg/mod/github.com/prometheus/client_golang@v1.7.1/prometheus/wrap.go:102 +0x195
github.com/prometheus/client_golang/prometheus/promauto.Factory.NewGaugeFunc(0x3b112a0, 0xc00097d440, 0x0, 0x0, 0x0, 0x0, 0x35151ca, 0x1b, 0x3555642, 0x2f, ...)
	/pkg/mod/github.com/prometheus/client_golang@v1.7.1/prometheus/promauto/auto.go:319 +0x13c
github.com/gocql/gocql.newPolicyConnPool(0x3ac0ba0, 0xc00097d3e0, 0x3b112a0, 0xc00097d440, 0xc0000b3400, 0xc0006901e0)
	/pkg/mod/github.com/grafana/gocql@v0.0.0-20200605141915-ba5dc39ece85/connectionpool.go:132 +0x22f
github.com/gocql/gocql.PoolConfig.buildPool(...)
	/pkg/mod/github.com/grafana/gocql@v0.0.0-20200605141915-ba5dc39ece85/cluster.go:28
github.com/gocql/gocql.NewSession(0xc00096cba0, 0x1, 0x1, 0x34de229, 0x5, 0x0, 0x77359400, 0x12a05f200, 0x2352, 0xc000ab46b8, ...)
	/pkg/mod/github.com/grafana/gocql@v0.0.0-20200605141915-ba5dc39ece85/session.go:136 +0x55f
github.com/gocql/gocql.(*ClusterConfig).CreateSession(...)
	/pkg/mod/github.com/grafana/gocql@v0.0.0-20200605141915-ba5dc39ece85/cluster.go:207
github.com/cortexproject/cortex/pkg/chunk/cassandra.(*Config).session(0xc000b762b8, 0x34e8b4f, 0xa, 0x3b112a0, 0xc00097d2f0, 0x0, 0x10bc238, 0xc000b764f8)
	/pkg/mod/github.com/cortexproject/cortex@v1.2.1-0.20200727121049-4cfa4a2978c2/pkg/chunk/cassandra/storage_client.go:125 +0x6bc
github.com/cortexproject/cortex/pkg/chunk/cassandra.NewStorageClient(0xc000ab46a0, 0xe, 0x2352, 0xc000ab46b8, 0x4, 0x34e0035, 0x6, 0x1, 0x10000, 0x0, ...)
	/pkg/mod/github.com/cortexproject/cortex@v1.2.1-0.20200727121049-4cfa4a2978c2/pkg/chunk/cassandra/storage_client.go:228 +0x88
github.com/cortexproject/cortex/pkg/chunk/storage.NewIndexClient(0xc000ab45f0, 0x9, 0x34e05ff, 0x6, 0x0, 0x4000000000000000, 0x4024000000000000, 0x0, 0x0, 0x186a0, ...)
	/pkg/mod/github.com/cortexproject/cortex@v1.2.1-0.20200727121049-4cfa4a2978c2/pkg/chunk/storage/factory.go:218 +0x9fa
github.com/cortexproject/cortex/pkg/chunk/storage.NewStore(0x34e05ff, 0x6, 0x0, 0x4000000000000000, 0x4024000000000000, 0x0, 0x0, 0x186a0, 0x3ff4cccccccccccd, 0x3ff0000000000000, ...)
	/pkg/mod/github.com/cortexproject/cortex@v1.2.1-0.20200727121049-4cfa4a2978c2/pkg/chunk/storage/factory.go:159 +0x675
github.com/grafana/loki/pkg/storage.NewStore(0x34e05ff, 0x6, 0x0, 0x4000000000000000, 0x4024000000000000, 0x0, 0x0, 0x186a0, 0x3ff4cccccccccccd, 0x3ff0000000000000, ...)
	/src/github.com/grafana/loki/pkg/storage/store.go:85 +0x116
github.com/grafana/loki/pkg/loki.(*Loki).initStore(0xc000179000, 0x3b5c8c0, 0xc0000c0e00, 0xc00097d020, 0xc00074e8f0)
	/src/github.com/grafana/loki/pkg/loki/modules.go:271 +0x33a
github.com/cortexproject/cortex/pkg/util/modules.(*Manager).InitModuleServices(0xc00023ee60, 0x34dcd10, 0x3, 0x3b554e0, 0xc0000e7680, 0xc0000e7680)
	/pkg/mod/github.com/cortexproject/cortex@v1.2.1-0.20200727121049-4cfa4a2978c2/pkg/util/modules/modules.go:87 +0x269
github.com/grafana/loki/pkg/loki.(*Loki).Run(0xc000179000, 0xc000b0c6c0, 0x4)
	/src/github.com/grafana/loki/pkg/loki/loki.go:178 +0x5b
main.main()
	/src/github.com/grafana/loki/cmd/loki/main.go:104 +0x61f

[codecov-commenter]

Codecov Report

Merging #2457 into master will decrease coverage by 0.05%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #2457      +/-   ##
==========================================
- Coverage   62.79%   62.73%   -0.06%     
==========================================
  Files         162      162              
  Lines       13963    13964       +1     
==========================================
- Hits         8768     8761       -7     
- Misses       4513     4518       +5     
- Partials      682      685       +3     

Impacted Files	Coverage Δ
pkg/loki/modules.go	5.02% <0.00%> (-0.03%)	⬇️
pkg/promtail/targets/file/tailer.go	73.86% <0.00%> (-2.28%)	⬇️
pkg/promtail/targets/file/filetarget.go	67.85% <0.00%> (-1.79%)	⬇️
pkg/logql/evaluator.go	92.47% <0.00%> (-0.41%)	⬇️