pump custom metrics default value

[Ambarish07]

Description

Prometheus custom metrics giving error when custom tags are introduced as part of custom metric labels.
Example : We had a usecase where we need a custom metrics where a custom Label should also be coming as part of prometheus custom metric. But if the custom label is not defined then the pump gives an error.
Solution : added a default value "UNKNOWN" of all the custom labels.

Related Issue

How This Has Been Tested

Tested in local by recreating tyk in local by adding custom metric.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

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