add context.db.instance to elasticsearch spans

[beniwohli]

The cluster name is taken as instance. This only
works for Elastic Cloud clusters, as they expose
the cluster name in form of a header.

Related issues

closes #1586

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• Start Time: 2022-10-26T13:31:08.386+0000

• Duration: 21 min 39 sec

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Test	Results
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Passed	5077
Skipped	3503
Total	8580

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Name	Metrics % (covered/total)	Diff
Packages	100.0% (68/68)	💚
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Classes	100.0% (230/230)	💚
Lines	92.185% (18436/19999)	👍 0.011
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