contrib: add db.system tag for all db integrations

[rarguelloF]

What does this PR do?

Add db.system tag for all db integrations.

Integration	db.system
bradfitz/gomemcache	memcached
database/sql	mysql/postgresql/mssql/other_sql
elastic/go-elasticsearch.v6	elasticsearch
garyburd/redigo	redis
globalsign/mgo	mongodb
go-pg/pg.v10	postgresql
go-redis/redis	redis
go-redis/redis.v7	redis
go-redis/redis.v8	redis
go.mongodb.org/mongo-driver/mongo	mongodb
gocql/gocql	cassandra
gomodule/redigo	redis
hashicorp/consul (KV only)	consul
oliviere/elastic	elasticsearch
syndtr/goleveldb/leveldb	leveldb
tidwall/buntdb	buntdb

Motivation

Part of the unified naming changes to standardize span tags across tracers.

Describe how to test/QA your changes

Updated existing tests.

Reviewer's Checklist

• If known, an appropriate milestone has been selected; otherwise the Triage milestone is set.
• Changed code has unit tests for its functionality.
• If this interacts with the agent in a new way, a system test has been added.

[pr-commenter]

Benchmarks

Comparing candidate commit 9781571 in PR branch rarguelloF/add-db-system-tags with baseline commit 863b9b2 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 6 cases.

[zarirhamza]

Missed testing with memcache, internal DSN sql test. Oliviere/elasticsearch test using wrong tag value. I fixed them tho, so no worries. LGTM