Improve the error messages of the metricdatatest package

[MrAlias]

After looking at the output, a few things stick out. Some of these are easy to tell what's wrong, but some less so: In the ResourceMetrics test: resources are easy to tell, but the scopemetrics less so:

For the ScopeMetrics, what would be a clearer way to you to see they are not the same for the ResourceMetric? I am trying to concisely state that the elements of the ScopeMetric slice differ, and showing what one contains but not the other, but I agree there is a lot of information there.

I think there are two solutions that would help:

1. Add formatting to the objects, like either a String() string function or some other formating, so we have just a bit less extra information.
2. Use a formatting function that is aware of the different types we have and can provide a more concise format.

My goal would be if presented with something like:

            ResourceMetrics ScopeMetrics not equal:
            missing expected values:
            metricdata.ScopeMetrics{Scope:instrumentation.Scope{Name:"A", Version:"", SchemaURL:""}, Metrics:[]metricdata.Metrics{metricdata.Metrics{Name:"A", Description:"A desc", Unit:"1", Data:metricdata.Sum{DataPoints:[]metricdata.DataPoint{metricdata.DataPoint{Attributes:attribute.Set{equivalent:attribute.Distinct{iface:[1]attribute.KeyValue{attribute.KeyValue{Key:"A", Value:attribute.Value{vtype:1, numeric:0x1, stringly:"", slice:interface {}(nil)}}}}}, StartTime:time.Date(2022, time.July, 20, 13, 18, 7, 260802950, time.Local), Time:time.Date(2022, time.July, 20, 13, 18, 8, 260802950, time.Local), Value:-1}}, Temporality:0x1, IsMonotonic:true}}}}
            unexpected additional values:
            metricdata.ScopeMetrics{Scope:instrumentation.Scope{Name:"B", Version:"", SchemaURL:""}, Metrics:[]metricdata.Metrics{metricdata.Metrics{Name:"B", Description:"B desc", Unit:"By", Data:metricdata.Gauge{DataPoints:[]metricdata.DataPoint{metricdata.DataPoint{Attributes:attribute.Set{equivalent:attribute.Distinct{iface:[1]attribute.KeyValue{attribute.KeyValue{Key:"B", Value:attribute.Value{vtype:1, numeric:0x1, stringly:"", slice:interface {}(nil)}}}}}, StartTime:time.Date(2022, time.July, 20, 13, 18, 7, 261802950, time.Local), Time:time.Date(2022, time.July, 20, 13, 18, 8, 261802950, time.Local), Value:2}}}}}}

and it only differed by one character I would be easy to tell apart.

Originally posted by @MadVikingGod in #3025 (comment)

[checkaayush]

@MrAlias I'd like to give this a shot. Apart from proposed suggestions, could something like https://github.com/davecgh/go-spew be used?

[MrAlias]

could something like https://github.com/davecgh/go-spew be used?

I'd rather not add an unneeded dependency to the package.