Remove unneeded locks in add_kubernetes_metadata

[jsoriano]

What does this PR do?

Read/write mutexes in matchers and indexers are only locked for read,
making them unnecesary. We could consider to add write locks, but writes
only happen on constructions, so it wouldn't be expected to read from these
structs before they are returned by the constructor.
These locks are acquired for any event that is enriched by the processor, so
unneeded locking may affect performance.

Also, refactor locking in indexing registry so it is used in all operations. It is
currently used only in some operations, and in external calls the lock needs to
be acquired in some cases.
This lock could be also removed because we only write to this structure
on init()s, but we use to make our registries thread-safe, so let's keep it
as is. It is only used when seting up the processor, and not when processing
events, so it shouldn't affect performance.

Why is it important?

Unneeded locking can be confusing and may affect performance.
Inconsistent locking may lead to unexpected results in future changes.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works (behaviour should be the same, covered by current cases)

[elasticmachine]

Pinging @elastic/integrations-platforms (Team:Platforms)

[blakerouse]

Looks good to me, but probably better to get another reviewer for +1.