Ensure that all query based scalers handles multiple triggers correctly

[zroubalik]

If there are multiple triggers in the ScaledObject, we need to be sure, that metricName used in GetMetricSpecForScaling() for each scaler returns unique metricName for the particualar ScaledObject. Most of the scalers should handle this correctly, as the metricName is usually based on a queue name or similar value. The metricName value should be immutable, eg. should be the same for the whole lifetime of the particular ScaledObject.

Query based scalers could be more problematic, because there's not an appropriate unique value that can be used as a base for the metricName, the query itself is usually quite long.

Some scalers are trying to handle this by allowing users to specify the metricName value, so they could distiguish the value manually (PostrgreSQL or InfluxDB).

Docs:
https://keda.sh/docs/2.1/scalers/postgresql/
https://keda.sh/docs/2.1/scalers/influxdb/

We should revisit all query based scalers and ensure that they handle this correctly and the same way both in the code and in the docs.
Relevant scalers:

• prometheus
• postgresql
• mysql
• influxdb
• mongodb

We might think about how to make this the most convenient for users as it could be, eg. try to find a solution without a need for specification of metricName manually, maybe a short hash of the query?

This PR is adding a check, that all metricNames per ScaledObject are unique: #1390

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