Add note to agent API docs warning tags

[felixbarny]

All agents should add a NOTE in the tag/label API to warn about a potential mapping explosion.

Suggested text (improvements are welcome, ofc.):

NOTE: Labels are indexed in Elasticsearch so that they are searchable and aggregatable.
By all means, you should avoid that user-specified data,
like URL parameters, is used as a tag key as it can lead to mapping explosions.

It probably makes sense to also link to https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping.html#mapping-limit-settings.

PR's:

• .NET - docs: clarify tags apm-agent-dotnet#197
• Go - docs: add tag information apm-agent-go#511
• Java - Add note that number and boolean labels require APM Server 6.7+ apm-agent-java#608
• Node.js - docs: add tip/warning to labels apm-agent-nodejs#1030
• Python - docs: clarify tags apm-agent-python#465
• Ruby - docs: add links to metadata section apm-agent-ruby#389
• RUM - docs: link to metadata section apm-agent-rum-js#254

[bmorelli25]

Thanks Felix. This also needs to be noted centrally in elastic/apm-server#2111. Once I have an idea of what that will look like, I'll take a better look at this suggested text and open PRs in all the agents.

[bmorelli25]

Now that elastic/apm-server#2129 is taking shape, I'm thinking the following for agent docs:

Labels/tags

TIP: Before using custom labels, ensure you understand the different types of
{apm-overview-ref-v}/metadata.html[metadata] that are available.

&

WARNING: Avoid defining too many user-specified labels.
Defining too many unique fields in an index is a condition that can lead to a
{ref}/mapping.html#mapping-limit-settings[mapping explosion].

Custom context

TIP: Before using custom context, ensure you understand the different types of
{apm-overview-ref-v}/metadata.html[metadata] that are available.

[bmorelli25]

PRs opened in all agents 🕺